The Biology Topics of ecology involve studying the relationships between living organisms and their environment.
Classification, Types, and Causes of Diseases
Health is the most important factor for the proper functioning and working of all living beings. A healthy individual can perform all their activities with the utmost efficiency and such individuals will prove to be an asset for whole of the society. Therefore, the significance of health is proved best by the proverb “Health is wealth”. But this ‘Wealth’ is lost when a person becomes infected by some diseases. So to avoid diseases our body has developed certain mechanisms like immunity. Such immunity may be natural or artificial. This chapter comprises the details of health, different diseases, and the methods to avoid and overcome such diseases.
Health is a state of the living body that is devoid of diseases and maintained with peace and joy. In the case of humans the World Health Organization.
The World Health Organization (WHO) has defined “health” as “a state of complete physical, mental, and social well-being and not merely an absence of a disease.” 7th April is celebrated as World Health Day. Thus health has three dimensions
- Physical: It is related to the “perfect functioning” of all the organs and systems of the body.
- Mental: It is related to a state of equilibrium and harmony between a person and his surrounding environment free from anxieties.
- Social: It is related to mutual understanding between family members, neighbours, and friends and a “feel good” factor with total security.
The term “disease” literally means “dis-ease”, i.e., “without ease” (uneasiness) or a feeling of being uncomfortable. It can thus be defined as the malfunctioning of the body with specific symptoms due to several factors leading to impairment of the health of that individual.
Types of Diseases
Human diseases are broadly grouped into the following four categories:
There are two broad categories of diseases, Congenital and Acquired.
1. Congenital Diseases:
These are present right from birth contributed by gene mutation, chromosomal aberrations, and environmental factors.
e.g., Colour blindness, Down’s syndrome, Cleft palate etc.
2. Acquired Diseases:
The diseases are contracted after birth, e.g., infection, degeneration, dirt, addiction, depression, cancer, etc.
Acquired diseases are of two types Communicable and Non-communicable.
1. Communicable or Infectious Diseases:
Caused by different disease-producing agents and can spread from individual to individual. As they develop only after birth, hence they are referred to as acquired diseases. Infectious disease is of many types-
- Viral disease: Transmitted through a virus, e.g., influenza, chicken pox, measles, mumps, AIDS, common cold, Covid-19, etc.
- Rickettsial disease: Transmitted through rickettsia, e.g., typhus fever, fever, trench fever, rocky mountain spotted fever, etc.
- Bacterial disease: Transmitted through bacteria, e.g., cholera, typhoid, etc.
Some Common Bacterial Diseases and the Name of Bacteria:
|Name of the Disease||Name of Bacteria (Causal Organism)|
|1. Diarrhoea||Bacillus coli|
|2. Dysentery||Shigella Dysenteriae or Bacillus Dycenteriae|
|3. Cholera||Vibrio cholerae|
|4. Diptheria||Corynebacterium Diphtheriae|
|5. Typhoid||Salmonella typhi or S. typhosa|
|6. Pneumonia||Diplococcus Pneumoniae|
|7. Anthrax||Bacillus anthracis|
|8. Tuberculosis||Mycobacterium tuberculosis|
|9. Rheumatic fever||Streptococcus sp.|
|10. Leprosy||Mycobacterium leprae|
|11. Plague||Pasteurella pestis|
|12. Tetanus||Clostridium tetani|
|13. Meningitis||Neisseria meningities|
|14. Whopping Cough||Bordello pertussis|
|15. Jaundice||Leptospira cetero-haemorrahagiae|
|16. Gonnorrhoea||Neisseria gonorrhoeae|
|17. Syphilis||Treponema pallidum|
|18. Enteric fever||Salmonella typhimurium|
|19. Abscesses||Staphylococcus aureus|
|20. Food poisoning||Clostridium botulinum|
|21. Gas gangrene||Clostridium septicum|
|22. Poisonous wound of skin and pharynx||Streptococcus pyrogens|
- Protozoan Disease: Transmitted through protozoan, e.g., malaria, sleeping sickness, etc.
Some Common Protozoan Diseases and their Causal Organisms:
|Name of the Disease||Name of Protozoa|
|1. Malaria||Plasmodium vivax|
|2. Amoebic Dysentery||Entamoeba histolytica|
|3. Kala-azar||Leishmania donovani|
|4. Sleeping Sickness||Trypanosoma gambiense|
|5. Giardiasis||Giardia intestinalis|
- Fungal disease: Transmitted through fungi, e.g., ringworm, athlete’s foot, Black fungus disease, etc.
- Helminthic disease: The disease caused by roundworms, tapeworms, e.g., filariasis, ascariasis, taeniasis, trichomonosis, etc.
Some Common Helminth Diseases and their Causal Orgamsms:
|Name of the Disease||Causal Organism|
|1. Teniasis||Taenia solium|
|2. Ascariasis||Ascaris lumbricoides|
|3. Filariasis||Wuchereria bancrofti|
|4. Ancylostomiasis||Ancylostoma duodenale|
2. Non-communicable Diseases:
Remain confined to the diseased person and do not spread or infect others by contact or carriers. For the same reason, they are also referred to as acquired diseases.
- Deficiency diseases: These diseases are caused by a deficiency of one or more nutrients. Kwashiorkor, marasmus (protein deficiency), scurvy (vit-C), rickets (vit-D), etc., are examples of deficiency diseases.
- Degenerative diseases: These are the diseases of old age caused due to the gradual decline in the functioning of body organs like the heart, lungs, arteries, joints, etc. Arthritis, gout, arteriosclerosis, etc., are these types of diseases.
- Allergies: These are caused due to hypersensitivity of the body to any foreign substances like dust, pollen, etc. Hay fever and asthma are examples of allergic diseases.
- Cancer: It is caused by uncontrolled divisions of cells in any part of the body to form a mass of cells, called tumour. There are several causes of cancer, like lung cancer due to smoking tobacco, mouth cancer due to chewing tobacco, and skin cancer due to exposure to UV rays.
- Mental Disorder: This can be related to depression, anxiety, mental disability, neurosis, psychosis, schizophrenia, etc.
- Addiction: These diseases are caused due to drug abuse, alcohol, and tobacco.
Epidemiology is the study of determinants, incidence or occurrence, distribution of health and disease, and possible control of diseases in a defined or specified population. Epidemiological studies enable one to quantity the degree upto which the risk factor and human intervention might affect a defined population in case of a medical crisis like an epidemic. The tools include surveys, analysis of data, rapid evaluation, surveillance, etc. It helps one to identify the etiology i.e., the cause of the disease, determine the extent to which the disease has affected the population, asses preventive and therapeutic steps that could be taken to keep the disease or the condition under control, to develop a public health policy to com¬bat or control the disease. The spectrum of occurrence of disease could be sporadic (occasional), endemic (regular), epidemic, and pandemic.
Nature of Diseases
Diseases are classified based on their occurrence, are as follows:
1. Endemic (Localized diseases):
When diseases occur only in certain localized areas affecting only a few persons, are called endemic diseases.
For example, goiter is found in the sub-Himalayan regions of India.
A disease is said to be epidemic when it breaks out and spreads rapidly from place to place affecting a large number of people.
For example, Plague assumed epidemic form in India (Surat) in 1997, killing several people.
A disease is called a pandemic when it spreads worldwide, for example, AIDS is a pandemic disease, and Covid-19 is also a pandemic disease.
When a disease occurs in scattered areas occasionally and individually.
Therefore, according to the nature of occurrence, human diseases may be classified into the following categories.
Disease Causing Agents
It is a substance responsible for the causation of disease either by being present in excess or deficient or absent. Among the five main disease agents (viz. biological, nutrient, chemical, physical, and mechanical), biological agents like viruses, bacteria, fungi, protozoans, helminths, insects, etc., are regarded as pathogens.
- Biological agents or pathogens: viruses, bacteria, protozoa, fungi, helminth, etc.
- Chemical agents:
- (a) Endogenous: Urea, uric acid, hormone, enzyme, etc.
- (b) Exogenous: Fumes, dust, pollens, metal etc.
- Physical agents: Heat, cold, humidity, pressure, sound, radiation, etc.
- Mechanical agents: Fractures, dislocation, injuries, etc.
- Nutrient agents: Minerals, carbohydrates, protein, vitamins, etc.
- Etiologic agents: Microorganisms that cause infection. The pathogenicity of an etiologic agent is its ability to cause disease.
The pathogenicity of an etiologic agent depends on virulence and invasiveness. Virulence refers to the severity of infection, that can be expressed by the incidence of disease and death rate, an example of a highly virulent organism is the etiologic agent of plague, Yersinia pestis.
Any infectious disease usually does not appear immediately after the infection, but it may take some time to appear which may vary from a few hours to a few days. Thus incubation period is the period between the entry of germs and the appearance of the first symptoms of the disease. All infectious diseases have a definite incubation period. An incubation period is the period between infection with the agent and the onset of symptoms of infection.
Incubation Period of Some Diseases:
|1. Pneumonia||1 – 3 days|
|2. Cholera||2 – 5 days|
|3. Typhoid||7 – 20 days|
|4. Tetanus||4 – 20 days or more|
|5. Dysentery||3 – 7 days|
|6. Diptheria||2 – 5 days|
|7. Measles||10 – 12 days|
|8. Chickenpox||14 – 21 days|
|9. Whooping cough||10 – 15 days|
|10. AIDS||Upto 15 years or more|
Transmission of Diseases
Communicable diseases are transmitted in two ways:
A. Direct Transmission of Diseases
Direct transmission of diseases is taken place by the following agents:
- Direct contact with an infected person, like chicken pox, smallpox, measles, or Covid-19.
- Droplet infection through sneezing, or coughing, e.g., common cold, influenza, pneumonia, diphtheria, tuberculosis, Covid-19, etc.
- Contact with soil as in tetanus.
- Animal bites as in rabies.
- Through the placenta as in syphilis.
B. Indirect Transmission of Diseases
The germs or pathogens of some diseases enter the body through some intermediate agents. These agents include Unclean hands and fingers may transfer pathogens of certain diseases, like ascariasis, and taeniasis.
Vector and its role in the transmission of disease :
- Carriers or vectors, e.g., female Anopheles the vector of malaria. House fly is the carrier of typhoid, cholera, dysentery, etc.
- Airborne as in the case of epidemic typhus.
- Vehicle-borne agents like ice, water, food, and blood carry pathogens of cholera, typhoid, AIDS, etc.
Differences between Infectious and Non-Infectious Diseases
|Infectious or Communicable Diseases||Non-infectious or Non-Communicable Diseases|
|1. They are caused by the attack of pathogens.||1. They are caused by factors other than living pathogens.|
|2. The diseases are brought about by extrinsic or external factors.||2. The diseases are mostly brought by intrinsic or internal factors.|
|3. Infectious diseases can pass from a diseased person to a healthy person.||3. Non-infectious diseases cannot pass from one person to another.|
|4. Transmission of infection occurs through direct contact or some medium (air, water, vectors).||4. Transmission is absent. However hereditary diseases are transmitted from parent to offspring.|
|5. Community hygiene can reduce the incidence of infectious diseases.||5. Community hygiene is ineffective in reducing the incidence of non-infectious diseases.|
|6. Examples: Cold, Cholera, Tuberculosis (T.B.), Pneumonia, Malaria, Chickenpox.||6. Examples: Diabetes, Hypertension, Goitre.|
Diseases are described as being acute, when their symptoms are quickly visible in the body e.g., common cold.
These types of diseases are long-term, with the symptoms lasting for months or years. Such diseases are called chronic diseases, e.g., elephantiasis, tuberculosis.
Differences between Acute and Chronic Disease
|Acute Disease||Chronic Disease|
|1. These last for only a short period.||1. These last for a long time, even as much as a lifetime.|
|2. These do not cause long-term bad effects on human health.||2. These cause drastic long-term effects on human health.|
|3. Examples include cold, cough, typhoid, cholera, etc.||3. Examples include elephantiasis, cardiovascular diseases, tuberculosis, diabetes, arthritis, cancer, etc.|
These diseases are present since birth. They are caused due to genetic abnormalities or metabolic disorders or malfunctioning of any organ. They are permanent, generally not easily curable, and may be passed on to the children.
These diseases are those which develop after birth. Acquired diseases can be broadly classified into two types:
- communicable or infectious diseases
- non-communicable or non-infectious diseases
These diseases are caused by some biological agents or pathogens such as viruses, bacteria, protozoans, helminths, nematodes, and fungi. Infectious diseases can rapidly spread from one person to another by various means such as by physical contact, water, air, food, and insects (vectors). Since these diseases are communicated from diseased person to healthy person they are known as infectious diseases.
These include the above-described organic or metabolic diseases. The non-infectious diseases are restricted only to those persons who are suffering from them. These are not spread from an infected person to a healthy person. Non-infectious diseases may be caused from:
- the lack of certain essential substances in our diet, e.g., proteins, vitamins, minerals (deficiency diseases)
- general wearing out or degeneration of tissues as in old age (degenerative diseases)
- uncontrolled growth of tissues in any part of the body (cancer)
- defects in the metabolic reactions (metabolic disorders)
- injury and damage to any part of the body by accidents.