- 1 Infectious Diseases – Symptoms and Causes and What is Vaccination?
- 1.1 Types of Infectious Agents
- 1.2 Means of Spread of Infectious Diseases
- 1.3 Some Infectious and Dangerous Diseases in Humans
- 1.4 (a) Covid 19
- 1.5 (b) Rhinitis or Common Cold
- 1.6 (c) Chickenpox
- 1.7 (d) Tuberculosis
- 1.8 (e) Typhoid
- 1.9 (f) Pneumonia
- 1.10 (g) AIDS (Acquired Immuno Deficiency Syndrome)
- 1.11 Symptoms of Disease
- 1.12 Principle of Treatment
- 1.13 Antibiotics
- 1.14 Principles of Prevention of Disease
- 1.15 Vaccination
The study of human anatomy and physiology is one of the crucial Biology Topics for medical professionals and researchers.
Infectious Diseases – Symptoms and Causes and What is Vaccination?
Types of Infectious Agents
Organisms that can cause diseases are classified into a wide range of categories. Some of them are viruses, some are bacteria, some are fungi and some are unicellular animals, the protozoans. Some diseases are also caused by multicellular organisms such as different kinds of worms.
Common examples of diseases caused by viruses are the common cold, influenza, dengue, fever, and AIDS. Diseases such as typhoid fever, cholera, tuberculosis, and anthrax are caused by bacteria. Many common skin infections are caused by different kinds of fungi. Protozoans cause many familiar diseases such as malaria and kala-azar. Worms tend to cause a variety of intestinal infections and elephantiasis.
All viruses live inside host cells, whereas bacteria very rarely do. Viruses, bacteria, and fungi multiply very quickly, but worms multiply very slowly in comparison. These common traits mean that many biochemical processes inside their cells are similar. As a result, drugs that block one of these biochemical mechanisms in one member of the group will be effective against many other members of the group. However, the same drug will not work against a microbe belonging to a different group.
For example, antibiotics commonly block biochemical pathways important for bacteria. Therefore, most broad-spectrum antibiotics work against many species of bacteria, rather than simply working against one.
Since viruses do not use these pathways at all, that is why antibiotics do not work against viral infection. For example, if we have a common cold, taking of antibiotics does not reduce the severity or duration of the disease. Instead, our body secretes an antiviral protein, called interferon to combat the virus of cold.
However, if we get a bacterial infection along with the viral cold, then taking antibiotics will help. In that case, too, the antibiotic will work against the bacterial part of the infection, not the viral infection.
Common Human Disease Caused by Infectious Agents
|1. Viruses||Common cold, Influenza, Dengue fever, Poliomyelitis, Hepatitis-B, AIDS (Acquire Immuno Deficiency Syndrome), Chicken pox, Measles, Mumps, SARS (= Severe Acute Respiratory Syndrome), Smallpox, Swine flue (HINI), Japanese encephalitis|
|2. Bacteria||Typhoid fever, Cholera, Tuberculosis, Anthrax, Acne, Tetanus, Food poisoning|
|3. Fungi||Athlete’s foot, Ringworm, and many other skin infections|
|4. Protozoa||Malaria, Kala-azar, Amoebic dysentery, Sleeping sickness|
|5. Worm||Intestinal worm infections, Elephantiasis|
Means of Spread of Infectious Diseases
Infectious diseases spread from one infected person to other normal persons by various methods.
1. Air-borne Diseases:
e.g., common cold, pneumonia, and tuberculosis. Such disease-causing microbes are spread throughout the air. The transmission of these microbes occurs through the little droplets coughed out by an infected person who sneezes or Coughs. A person in the vicinity of such a person can inhale these disease-causing microbes and may become infected.
Diagram showing the potential risk of getting air-transmitted diseases as we move closer to the infected person. In closed areas, the droplet nuclei recirculate and pose a risk to everybody. Thus, overcrowded and poorly ventilated housing is a major factor in the spread of airborne diseases.
2. Water-borne Diseases:
e.g., cholera. Infectious diseases can also spread through water. These occur when a stool from someone suffering from an infectious gut disease, such as cholera or amoebiasis, gets mixed with the drinking water used by people living nearby. The cholera-infested bacteria can enter new hosts through the water they drink and can cause disease in them.
3. Sexually-Transmitted Diseases:
e.g., Syphilis and AIDS. Both of these pathogens are transmitted by sexual contact from one partner to the other. However, such sexually transmitted, diseases are not spread by casual physical contact. Casual physical contacts include handshakes, hugs, sports such as wrestling, or by any of the other ways in which we touch each other socially.
4. Formite-borne Diseases:
Articles coming in contact with patients act as sources of infection, e.g., door handles, taps, garments, currency, utensils, crockery, etc.
5. Spread of Disease through Vectors:
Many animals which live with us may carry diseases. These animals can transfer infecting agents from a sick person to another potential host. Thus, these animals act as intermediaries or vectors. Vectors are carriers of a disease or infection. Mosquitoes (Anopheles) are vectors of a disease, called malaria. In many species of mosquitoes, the females need highly nutritious food in the form of blood in order to be able to lay mature eggs. Hence, they feed on many warm-blooded animals including humans.
Common Human Diseases Transmitted by Insects
|Insect Vector||Diseases Transmitted|
|(i) Anopheles (Female)||Malaria|
|(iii) Aedes||Yellow fever, Dengue, Filariasis|
|(i) Housefly||Typhoid, Diarrhoea, Dysentery, Cholera, Tuberculosis, Conjunctivitis (an eye disease)|
|(ii) Sandfly||Kala-azar, oriental sore|
|(iii) Tse-tse fly||Sleeping sickness|
|3. Louse||Epidemic typhus|
|4. Rat flea||Bubonic plague|
Thus, means of transmission (spread) of infectious diseases may be of two main types:
- Contact with an infected person (e.g., AIDS)
- Contact with soil (e.g., Tetanus)
- Animal bites (e.g., Rabies)
- Transplacental (e.g., AIDS, German measles, and Syphilis)
- Through vectors (e.g., Malaria)
- Through contaminated food and water (e.g., Amoebiasis, Hepatitis, etc.)
- Air-transmitted disease (e.g., common cold, T.B., pneumonia)
- Formite borne
Some Infectious and Dangerous Diseases in Humans
(a) Covid 19
Causative Agent: Corona Virus.
Viral Structure: Oval in structure with a protein coat containing spike protein that is important for entry into the cell. Inside the coat, there is a single-stranded RNA genome. The genome replicates inside the body cell and progeny virus particles are formed.
Target cell: Cells of the lung and therefore lung is the most affected organ.
Symptoms of Infection of Covid 19:
- Loose motion
- Disappearance of taste and smell.
- Soar throat.
- Shortness of breadth.
- Dry cough.
- Nasal congestion.
- Muscle soreness.
- In severe situations organ failure.
Mode of Transmission of Covid 19:
When healthy persons came into contact with covid affected ones, the disease infects the healthy persons and spreads rapidly. The droplets from an infected Covid patient, carry the causative agent of coronavirus. Therefore, any person getting the droplets from an infected person may develop the disease. Experts suggest that virus-carrying droplets may also be transmitted through air as aerosol.
Detection of Infection of Covid 19:
For the treatment of Covid, patient detection is very important. As there is no specific treatment for the disease, the only symptomatic remedial measure is the treatment. In some infected persons, initially, no severe symptoms of Covid 19 may be observed. These persons are called as asymptotic covid patients. Their infection may be detected by proper detection methods. This method is called the RT-PCR test. It is a coronavirus genome-based detection method. It is a little time taking method, but authentic, and getting positive results in RT-PCR test is sure proof of coronavirus infection. If infection is detected, the treatment may be continued in home isolation. Only in case of severe 0, depletion (When the O2 saturation level of the body is below 94%) hospitalization of the patient is necessary. Another test is rapid antigen testing which is usually completed within Vi an hour. This is not that much authentic as RT-PCR. On the basis of Covid-19 symptoms and RT-PCR testing, if the patient is treated in a rapid fashion, the patient may be rescued.
Prevention of Covid 19:
Prevention of Covid 19 is not too difficult. According to WHO guidelines, some prophylactic measures may be taken by non-affected common men. These measures are
- Avoidance of contact with affected covid patients.
- Use of N95 mask.
- Maintenance of social distancing i.e., the distance between two persons should be a minimum of 6 ft.
- Avoid gathering and mixing with many persons.
- Washing of hands with soap from time to time.
- Use of sanitizer after use of hands for a carriage and contact.
- Sneezing and coughing covering mouth and nose with tissue paper or some cloth like a handkerchief.
- Avoidance of mixing with pets.
- Avoidance of hand-shaking and embracing other persons.
- If a person appears to be affected by coronavirus, he should immediately be isolated from others in the same house.
Besides, the vaccination against Covid is also recommended for its prevention. In our country, three vaccines have been developed, Covaxin, Covishield, and Covovax. Two doses of any of the three vaccines may be taken at 4-6 weeks intervals.
Treatment for Covid 19:
It has already been mentioned that there is no specific treatment for Covid 19. Only symptomatic treatment may be done with the consultation of a doctor. But the treatment in home isolation is always may not be possible. Especially if the patient’s O2 saturation level falls (go below 94%) then hospitalization is very essential and the treatment is done with O2 support under the survillence of qualified doctors.
(b) Rhinitis or Common Cold
It is also a viral disease and the most common infectious disease in humans. The virus causing rhinitis is called Rhinovirus. On infection, the disease causes symptoms such as
- Nasal congestion
- Cough and sneezing
- Mild fever
- Sore throat
- Flow of mucous
- Inflammation of the respiratory passage.
Transmission of Rhinitis:
Mainly through contact this disease is transmitted from one person to the other. Droplets from the patient carry the virus for the disease. The use of materials from a patient or contact with him spreads the disease among others.
Prevention of Rhinitis:
The disease is not fatal and therefore no strict measure for its prevention is required. Without taking any medicine the disease is cured after one week. The spread of rhinitis may be prevented by avoidance of contact with the infected person.
Chickenpox is a highly infectious disease caused by the varicella-zoster virus. Due to the virus infection, itchy blister-like rashes appear on the skin. After exposure to the virus after 10 to 21 days the sign and symptoms of the disease appear.
Symptoms of Chickenpox:
- Loss of appetite
- The appearance of pink to red bumps over the skin especially on the chest, back, and face.
- Then conversion of bumps into fluid-filled blisters which break and then leak.
- The blisters continue to appear for several days.
- The rashes are accompanied by dizziness disorientation, rapid heartbeat, shortness of breath, tremors, loss of coordination, worsening cough, vomiting, and stiff neck.
Transmission of Chickenpox:
Chickenpox is transmitted from an infected person directly by touching blisters, saliva, or mucous of an infected person. The virus may also be transmitted through the air by sneezing and coughing.
Prevention of Chickenpox:
For preventing the spread of this disease, the healthy person should avoid any type of contact with the infected person. There is a vaccine for chicken pox. The healthy person may innoculate them with vaccines to prevent infection. Above all maintenance of personal hygiene and avoidance of contact with the infected person is of prime importance. If some person in a house becomes affected with chicken pox he should first be isolated in the house. Other noninfected persons should be vaccinated. The healthy persons in the house avoid contact with the infected as far as possible.
Treatment for Chickenpox:
No specific treatment is required. As per the advice of the doctor, some medicines may be taken. Chickenpox is not fat. It is cured after a few days.
It is also an infectious and bacterial disease. Mixing with a patient and sharing the clothes and articles may infect one healthy person.
Causative agent: Mycobacterium tuberculosis
Symptoms of Tuberculosis:
- Fever (not very high) with chills.
- Cough lasting more than 3 weeks and coughing up blood.
- Mucous secretion with blood.
- Weakness and night sweating.
- Weight loss.
- Lungs are seriously affected.
- Chest pain.
- Loss of appetite.
Detection of Tuberculosis:
Tuberculosis may be detected by proper tests. Sputum tests by the pathologist help to test the infection of tuberculosis. Besides Mantoux test is recommended for tuberculosis (Skin and blood test).
Transmission of Tuberculosis:
The disease is contagious in nature. Mixing with an infected person increases the chance of transmission of the pathogen. Through cough, sneeze, and droplets the pathogenic bacteria may go to the noninfected person. There a person should be cautious about handling a tuberculosis patient.
Prevention of Tuberculosis:
Maintenance of personal hygiene is the main thing behind the prevention of the disease. The person known to be infected with tuberculosis must be kept isolated. A healthy person may take vaccination against tuberculosis.
Treatment for Tuberculosis:
Tuberculosis is curable. Proper medical advice with the intake of antibiotics and a rich food supply appears to remedy tuberculosis.
Typhoid is an acute and most common communicable disease in our country.
Pathogen: Bacteria Salmonella typhi is the causal organism of typhoid.
The disease occurs throughout the year but the frequency is higher during the rainy season.
Incubation Period: 10 – 15 days.
Transmission of Typhoid:
The disease is transmitted by the following ways:
- Spreads chiefly by ingestion of contaminated food, water, milk, and unwashed raw vegetable.
- The typhoid bacteria are excreted in the stool and urine of patients. Defecation and urination in the open lead to the spread of infection through the soil, food, water, and flies.
- Spreading also takes place through direct contact.
Symptoms of Typhoid:
- Characterized by continuous fever which increases day by day (ladder-like).
- The temperature rises more in the afternoon than in the morning.
- Fever is accompanied by headache and body aches.
- Constipation occurs.
- Sometimes a rash develops on the trunk at the end of the first week.
- In untreated cases, the patient may develop hemorrhage from the ulceration in the small intestine.
Diagnosis of Typhoid:
The widal test shows typhoid bacteria.
Prevention and Control of Typhoid:
- The patient, being a source of infection, should be isolated and his urine and stool are to be collected in a closed container and disinfected before disposal by flushing or burning.
- Patients’ personal clothing, bedclothes, etc., should be disinfected by boiling.
- Attendants of the patient should disinfect their hands.
- Proper community sanitation, safe water supply, and food sanitation should be ensured.
- Breeding of flies should be prevented by the use of insecticides and wire mesh doors and window doors.
- Immunization with the TAB vaccine provides protection against typhoid for three years.
- Antibiotics like chloramphenicol, ampicillin, etc., are used to cure the disease.
Pneumonia is a serious disease of the lungs.
Pathogen: Gram-positive bacteria Diplococcus pneumoniae.
Types of Pneumonia:
Pneumonia is of two types Broncho pneumonia and Lobar pneumonia.
- Broncho pneumonia: Children are affected. Bronchus, bronchiole, and lungs are affected.
- Lobar pneumonia: The lobes of the lungs are affected and become rigid.
Incubation period: 1 – 3 days
Modes of Transmission of Pneumonia:
The disease spreads by the sputum of the patient.
Symptoms of Pneumonia:
- Serious changes in lungs manifested by mucous or fluid accumulation in alveoli and bronchiole.
- Breathing difficulty.
- Chest pain.
- Abdominal distension.
- Headache and sudden chill.
- Rapid breathing.
- Bluish and greyish colour of lips and fingernails due to deficiency of oxygen in the body.
- Death if untreated.
Diagnosis of Pneumonia:
Chest X-ray, complete blood count test, CT scan of the chest, bronchoscopy, and pleural fluid culture.
Control of Pneumonia:
Use of antibiotics like erythromycin, tetracycline, azithromycin, etc.
(g) AIDS (Acquired Immuno Deficiency Syndrome)
It is a life-threatening disease due to HIV (Human Immunodeficiency Virus). HIV was discovered by Luc Montagnier of France in 1983 and Robert Gallo of America in 1984. It is an RNA virus of the sub-group Lentivirus of Group Retrovirus. HIV contains reverse transcriptase enzyme which forms DNA from RNA. AIDS was first identified in America and since then it is recognized as the second life-threatening disease in the world. According to statistics from the World Health Organisation (WHO) in 2000, about 36 million human beings are attacked by AIDS in the whole world. AIDS is more in African countries. In West Bengal AIDS was identified in 1986.
Statistics show that young boys and girls are attacked more by AIDS nearly in equal proportion. Homosexual males and commercial sex workers are also attacked more.
Causative Organism: A retrovirus named, HIV or Human Immuno Deficiency Virus.
Incubation Period: 2 – 3 years, upto 10 years.
Structure of HIV Virus:
- HIV is spherical or rounded and about 90-120 nm in diameter.
- The capsid is made up of helically spiral capsomeres.
- The capsid is enclosed in a protein shell, called an envelope.
- The envelope contains spikes, made up of glycoprotein.
- In the core region, there are two identical filaments of RNA and reverse transcriptase enzyme.
Mode of Transmission/Infection of HIV:
- Through sexual contact.
- Transfusion of HIV blood.
- Use of the same injection syringe for different persons without sterilization.
- From the suffering mother to the embryo through the placenta.
- Artificial insemination in the uterus.
- From suffering mother’s milk to the baby.
- Use of the same blade or razor in the saloon for different persons.
- Transplantation of tissue from one to another person.
Effects of HIV Infection:
- HIV after entering into the blood attacks T lymphocyte (CD4) cells. As a result, the immunity of the body is reduced.
- Attacked persons are not easily cured of other infections by viruses, bacteria, protozoa, etc.
Symptoms of AIDS:
The symptoms are expressed in three stages:
A. Primary or Acute Stage:
It starts after 3-4 weeks of infection and lasts for 2 weeks. The symptoms are as follows:
- Fever, fatigue, ulcer in the throat, and commonly lymph adenopathy.
- Rash at trunk, palm, and sole.
- Low WBC count in the blood.
- The appearance of HIV antibodies after 3-4 weeks of infection.
B. Middle, Latent Stage:
It may last for a long time. The symptoms are as follows:
- The patients are asymptomatic with the absence or presence of less virus in the blood.
- HIV is formed in the lymph node.
- AIDS-related complex (ARC) is seen.
- Continuous fever with fatigue.
- Loss of body weight with lymph adenopathy.
C. Late Immuno Deficiency Stage:
The symptoms are as follows:
- Reduction of CD4 cells (400 per pm)
- Pneumocytosis carinii pneumonia disease
- Kaposis sarcoma
- Isolated cerebral lymphoma
- Candidiasis (Injury at mouth, pharynx, and oesophagus)
Diagnosis of AIDS:
The presence of HIV antibodies in the blood is identified by the screening tests. This is done by
- ELISA (Enzyme-Linked Immuno Sorbent Assay)
- Western Blot Test.
Prevention of AIDS:
The following measures should be taken to prevent AIDS:
1. Prevention of Transmission Through Sexual Connection:
- The male should use a condom during sexual intercourse.
- It is better to avoid sexual contact with commercial sex workers.
2. Prevention of Blood Borne Transmission:
- The presence of HIV antibodies in the blood Blood Bank should be tested before use. Transfusion of HIV-positive blood must be avoided.
- The same syringe should not be used to collect blood from more than one person.
3. Precautions of Health Workers and Surgeons:
- The health workers should use gloves in hand to work with blood, other body fluid, semen, etc., in the laboratory.
- Surgeons should always use gloves on their hands. The knife, scissors, blade, etc., instruments must be sterilized before operation.
4. Public Education:
- The public should be made aware of the mode of transmission of AIDS and its bad effects.
- The public should uplift their living standard to avoid AIDS.
5. Vaccine: No vaccine has yet been discovered to prevent AIDS.
6. Medicine: The first medicine to control AIDS is Zidovudine (ZDV/AJT). This medicine has been effective to neutralize the symptoms of AIDS and to reduce the rate of death. The medicines in the next stages are Lamivudine (3TC), Didanosine (dd), Zalcitabine (ddc), Stavudine (d4T), and Abacavir. Each of these medicines competes with a natural nucleoside.
Symptoms of Disease
Point of entry and place of infection of microbe inside the human body: Our body is quite large in comparison to any type of microbe (which is a parasite and pathogen). So there are many possible areas, organs, or tissues within our body where microbes may reside. Different species of microbes appear to have evolved to move to different parts of the body. In part, this selection is related to their point of entry.
For example, if the microbes enter the air via the nose, they are likely to go to the lungs. This happens in the case of bacteria that cause tuberculosis (T.B.) of the lungs. If the microbes enter through the mouth, they can stay in the lining of the gut as do typhoid-causing bacteria. These microbes can also go to the liver, like the viruses that cause jaundice (Hepatitis B).
However, this pattern is not followed by all microbes. For example, infection of HIV takes place via the sexual organs but it tends to spread to lymph nodes all over the body. Likewise, malaria-causing protozoan enters through a mosquito bite but it first goes to liver cells and then to red blood corpuscles (RBCs). Similarly, the virus causing Japanese encephalitis, or brain fever, enters through a mosquito bite. But it goes to infect the brain.
Symptoms and Signs: Symptoms are evidence that points to the presence of diseases. They are visible in the form of structural and functional changes in the body or body parts. They indicate that there is something wrong with the body.
On the basis of the symptoms of a disease, physicians search for definite clues or signs of the disease. For this, they use certain instruments and conduct laboratory tests (e.g., tests in pathologists’ labs) to pinpoint the cause of the disease.
Differences between Symptoms and Signs
|1. They indicate the presence of disease.||1. They provide information about the presence of a particular disease.|
|2. Symptoms are a collective indication of a number of diseases in a particular part or organ.||2. They are distinct for different diseases.|
Besides the various tissue-specific effects of the infectious disease, there can be other common effects also. Most of these common effects of the disease depend on the ability of a patient’s immune system to become activated in response to an infection. An active immune system recruits many cells to the affected tissue to destroy the disease-causing (i.e., pathogenic) microbes. Such a recruitment process is called inflammation. Local effects of inflammation are shown in the form of swelling and pain. The general effect of inflammation is depicted in the form of fever.
Inflammation or Oedema
Inflammation is due to the escape of chemicals, including histamine and 5′-hydroxytryptamine from the damaged tissues. This increases the amount of blood in the wounded area and raises the temperature locally. Leaking from the capillaries is also increased, permitting the discharge of plasma and white blood cells into the surrounding tissues. The consequent swelling of the area is called oedema. This plasma contains chemicals, which inhibit the growth of bacteria or kill them, and antibodies and phagocytes (neutrophils, monocytes or macrophages, etc.) combat the spread of infection. One of the chemicals is interferon which is secreted mainly by macrophages and some other white blood cells if they are exposed to foreign antigens. Interferon makes body cells resistant to infection by viruses.
Infection of HIV is found to have a multiple-dimensional effect. In the case of HIV infection, the virus goes to the immune system and ultimately damages its function. Thus, many symptoms of HIV-AIDS infection are due to the fact that the patient’s body can no longer fight off many minor infections that he faces every day. For instance, in an AIDS patient, every small cold can enhance pneumonia. Similarly, a minor gut infection can produce major diarrhea with blood loss. Thus, it is these secondary infections that kill people suffering from the HIV-AIDS.
One point should also be clear to us that the severity of disease symptoms depends on the number of microbes in the body. For example, if the number of microbes is very small, then the symptoms of the disease will be minor or unnoticed. However, if the numbers of the same pathogen/microbe are large, the disease can be severe enough to be life-risking. The number of microbes/pathogens is mainly determined by the immune system.
Principle of Treatment
There are two ways to treat an infectious disease. One is to reduce the effects of the disease and the other way is to kill the cause of the disease. For the first requirement, we can provide treatments that will reduce the symptoms. The symptoms are usually a result of inflammation. For example, we can take medicines that bring down fever, and reduce pain or lose motion. We can take bed rest so that we can conserve our energy. This exercise will enable us to focus on the healing.
However, such a kind of symptom-directed treatment by itself is inadequate. Since it will not make the pathogen go away, the disease will not be cured. For that, we have to kill the microbe itself.
How to kill microbes? Pathogenic microbes can be killed by two common methods. One way is to use medicines that kill microbes. In fact, we have to choose a drug that may work only on the parasite or pathogen. If that drug affects the host’s metabolism, then our very purpose of treatment will be defeated.
We have seen earlier that microbes can be classified into different categories. Thus, they are viruses, bacteria, fungi, or protozoa. Each of these groups of organisms will have some essential biochemical life processes which are peculiar to that group and not shared with the other groups. These processes may be pathways for the synthesis of new substances (=anabolism) or respiration (=catabolism). For example, our cells may make new substances by a mechanism different from that used by pathogenic bacteria. We have to find a drug that blocks the bacterial synthetic pathways but does not affect our own cellular pathways. This is what is achieved by antibiotics. Similarly, there are drugs that kill protozoa such as malarial parasites.
1. Antibiotics are chemicals produced by microorganisms (mainly bacteria and fungi) which at low concentration levels, have the ability to destroy or inhibit the growth of pathogens. The first antibiotic was penicillin, which was developed in the 1940s in response to the need to treat soldiers in the Second World War. There are now about 50 to 100 commercially available antibiotics.
2. Antibiotics are characterized by their range of effectiveness and their mode of action against pathogens:
- Broad-spectrum antibiotics kill a wide range of bacteria.
- Narrow-spectrum antibiotics are effective against only a few types of bacteria.
To kill a specific pathogen, you have to use a narrow spectrum of antibiotics that is specific to the disease.
3. All antibiotics must have selective toxicity. This means they should kill or inhibit the growth of bacteria or fungi, but cause little or no damage to the host. The drug zidovudine (AZT) stops HIV from replicating itself by binding with a reverse transcriptase enzyme and blocking its action. But it has a side effect, i.e., it causes anaemia.
Principles of Prevention of Disease
The following three limitations are normally confronted while treating an infectious disease:
- Once someone gets a disease, his or her body functions get damaged and may never recover completely.
- Treatment of a disease takes time. This means that if someone is suffering from a disease, he is likely to be bedridden for some time, even if he is given proper treatment.
- The person suffering from an infectious disease can serve as the medium for further spread of infection to other people. Hence, the prevention of diseases is better than their cure.
General Ways of Prevention of Infectious Diseases:
Public hygiene is one basic key to the prevention of infectious diseases. Thus, in this method of prevention of diseases, the following practices are adopted:
- To avoid exposure to air-borne microbes, adopt living conditions that are not overcrowded.
- To prevent exposure to water-borne microbes, safe drinking water should be provided.
- To avoid vector-borne infections, we can provide a clean environment as it would not allow mosquito breeding.
Specific ways of Prevention of Infectious Diseases:
If someone is suffering from a cold and cough in the class, it is likely that the children sitting around will be exposed to the infection. But not all of them will catch the infection. This may be due to a strong immune system.
Immune System and Immunisation:
Children usually escape cold and cough infections because their immune system (involving certain white blood cells) normally fights off the microbes (pathogenic microorganisms). These cells (i.e., lymphocytes) become active every time disease-causing microbes enter the body.
An immune response is the way in which a human body responds to invasion by a specific pathogen or antigen. This response involves the production of cells (lymphocytes) and chemicals (antibodies), that are designed to defend the body against the pathogens.
Antigens are substances that can activate an immune response. Antigens trigger the production of antibodies (glycoproteins or immunoglobulins) by the immune system. Each type of antibody is specific to a particular antigen and reacts with it to render it harmless.
There are many different types of white blood cells involved in immunity. Cell-mediated immunity involves T-Iymphocytes and macrophages. Macrophages are phagocytic, i.e., they engulf and digest all types of foreign cells and viruses.
Lymphocytes are white blood cells that recognize and react with antigens. They are of two types: T- lymphocytes, and B-lymphocytes. T-lymphocytes become activated in the thymus gland and can be of four types:
- Cytotoxic T-cells or killer cells
- helper T-cells
- memory T-cells
- suppressor T-cells
This involves the production of B-lymphocytes which are activated by antigens attached to the macrophage membrane. B-lymphocytes are of three types:
- Plasma B-cells: They secrete antibodies into the blood circulation.
- Memory B-cells: They do not produce antibodies but become programmed to remember a specific antigen and respond very rapidly to any subsequent infection.
- Dividing B-cells: They produce more B-lymphocyte cells.
The reaction between Antibody and Antigen:
It involves the following three steps:
- The antibody becomes attached to the antigen at the antigen-binding sites like a key in a lock.
- This causes the antibody to change from a T-shape to a Y-shape.
- Antibody causes the antigen to stick together, a process called agglutination, which stimulates phagocytosis by neutrophils.
1. Active Immunity:
The pathogen invades the body and the body responds by stimulating the production of T-lymphocytes and B-lymphocytes, both of which are involved in the immune response. Memory cells are formed which provide long-term immunity to an antigen.
This type of immunity is termed active immunity because the lymphocytes get activated by antigens present on the surface of the pathogen. Since this activation takes place during the natural course of an infection, this is called natural active immunity.
An immune response can also be triggered artificially. This involves the injection of antigens into the body. We call this artificial active immunity, although it is more commonly referred to as vaccination.
2. Passive Immunity:
It occurs when an individual becomes temporarily immune to an antigen by receiving ready-made antibodies from someone else. Passive immunity is of two types:
- Natural Passive Immunity: It occurs when pre-formed antibodies pass naturally from mother to baby across the placenta and in breast milk. Immunity is only temporary since a baby’s body does not ‘know’ how to make more antibodies. But it provides the baby with protection until it develops its own immune system.
- Artificial Passive Immunity: It occurs when pre-formed antibodies extracted from one individual, are injected into another as serum. This sort of immunity can provide a “quick fix” and is given to people who for example have been bitten by poisonous snakes or rabid dogs.
Normally, immune cells (WBCs) manage to kill off the infection long before it assumes major proportions. If the spread of the infecting microbes is controlled, the symptoms and effects of the disease will be minor. In other words, exposure to infection or an infectious microbe does not necessarily imply that symptoms are not noticeable.
So one way of looking at severe infectious disease is that it represents a failure of the immune system. The functioning of the immune system, like any other system in the body, will not be good enough if proper and sufficient nourishment and food is not available. Therefore, a second basic principle of prevention of infectious disease is the availability of proper and sufficient food for everyone.
These days, there is no reported case of smallpox (A highly infectious viral disease) anywhere around the world. But as recently as hundred years ago, smallpox epidemics were quite prevalent. However, it was identified that patients who had contracted smallpox once developed immunity to that disease for life. So contracting a viral disease once meant prevention from subsequent attacks of the same disease.
When the immune system first encounters an infectious microbe, it responds against it and then remembers it specifically. So the next time that particular microbe or its close relatives enter the body, the immune system responds with greater vigour. This eliminates the infection even more quickly than the first occurrence of the disease. This is the basis of the principle of immunization.
Derivation of term vaccination: Three centuries ago, an English physician Edward Jenner (1749-1823) realized that milkmaids who had cowpox, did not catch smallpox even during epidemics as they had become resistant to the smallpox virus. This was because the smallpox virus is closely related to the cowpox virus. ‘Cow’ is ‘vacca’ in Latin, and cowpox is ‘vaccinia’. From these roots, the word vaccination came into usage.
How are Vaccines made?
A vaccine is an antigen that is injected or swallowed. It causes the development of active immunity in the patient. Small quantities of antigen introduced into the person’s body stimulate, the production of antibodies as if the body was infected by the disease. This type of immunity is long-term since the body develops memory cells in the natural pathway.
Antigens are treated before being introduced to the body of the person, in order to make them relatively harmless. Most vaccines are made in one of the following ways:
- Killed virulent organisms, e.g., vaccines for whooping cough bacteria. In this case, the bacteria are killed by heat or the use of chemicals, which denature their enzymes. So the dead pathogen will not cause the disease, but it will possess antigenic sites on its surface that will be recognized by T- and B- lymphocytes, and result in the production of antibodies in the recipient. But there is no chance of the pathogen replicating and causing infection.
- Live non-virulent strains, e.g., rubella. Vaccines made in this way are often called attenuated (= weaken) vaccines. In such vaccines, the pathogen is deliberately weakened to, ensure that it does not cause severe infection. Other examples are the BCG vaccine used against tuberculosis and the Sabin vaccine used against poliomyelitis, which is taken orally.
- Modified toxins, e.g., vaccines used against diphtheria and tetanus. In this type of vaccine, the toxoids (toxic substances) produced by the bacteria are made harmless. Toxoids are used to stimulate antibody production, but there is no risk of infection by the pathogen.
- Isolated antigen, e.g., influenza. Sometimes important antigens are separated from the microorganism, in this case by breaking up the pathogen’s structure and obtaining glycoproteins. The “flu vaccine” contains a mixture of antigens from various strains of influenza virus, in an attempt to combat the great variations that exist. This antigenic variation occurs in microorganisms that have a high mutation rate.
- Genetically engineered antigens, e.g., hepatitis B. In this most modern type of vaccine, restriction endonuclease enzymes are used to extract from the pathogen the genes that code for a particular antigen. Such genes are inserted in a harmless plasmid vector using a ligase enzyme. The bacterial cells then replicate to produce large amounts of antigen.
In this way, we ‘fool’ the immune system into developing a memory of a particular infection, by injecting something that mimics the microbe we want it to vaccinate against, into the body. This does not cause the disease but helps prevent any subsequent exposure to the infecting microbe from turning into an actual disease. There are vaccines against
- Whooping cough
Important Vaccines for Infants and Children
|Vaccine||Disease||Age group||Safety Level|
|1. DPT-Hib||Diphtheria (Corynebacterium diphtheriae), Tetanus, Pertussis (whooping cough), and Haemophilus influenzae type B.||To all infants of 1 1/2, 2 1/2, and 3 1/2 month age.||90% – 99%|
|2. Hepatitis-B||Hepatitis (serum hepatitis)||All infants, children, and even adults.||Not yet confirmed.|
|3. Polio||Poliomyelitis||All infants up to 5 years of age; minimum of three doses at a one-month interval||Nearly 100%|
|4. BCG||Tuberculosis||All children between 10 to 14 years.||Nearly 70%|