Contents
One of the most fascinating Biology Topics is the study of genetics and how traits are passed down through generations.
Classification, Types, and Causes of Diseases
Human health is affected by various factors, causes, or sources. These factors are of the following types:
Intrinsic or Internal Factors
The disease-causing factors that exist within the human body are called intrinsic factors. The important intrinsic factors which affect human health are the following:
- Malfunctioning or improper functioning of various body parts such as heart, kidney, liver, etc.
- Genetic disorder
- Hormonal imbalances
- Malfunctioning of the immune system of the body, e.g., allergy.
A few common diseases are due to hyposecretion and hypersecretion of hormones.
| Disease | Hormone involved | Level of hormone |
| 1. Grave’s disease (Exophthalmic goiter) | Thyroid hormones | Hypersecretion |
| 2. Cretinism (in infants) | – | Hypersecretion |
| 3. Myxoderma (in adults) | – | Hypersecretion |
| 4. Addison’s disease | Aldosterone | Hyposecretion |
| 5. Diabetes mellitus | Insulin | Hyposecretion |
The diseases caused by intrinsic sources are called organic or metabolic diseases.
Some of the diseases caused by intrinsic sources or factors are:
- Cardiac failure (Heart attack)
- Kidney failure
- Osteoporosis
- Myopia
- Cataract
- Sickle cell anemia
- Haemophilia
- Dwarfism
- Gigantism
- Cretinism
- Diabetes
- Allergies (e.g., asthma)
- Arthritis
- Cancer
Classification of Some Common Diseases of Humans
| Type of Disease | Causes of Disease | Example |
| 1. Physical | 1. Temporary or permanent damage to body parts. | 1. Bone fracture, leprosy. |
| 2. Infectious | 2. Invasion of the body by other organisms (viruses, bacteria, fungi, protozoans, nematodes, insects, etc.) | 2. Typhoid, diarrhea, malaria, hepatitis, AIDS, T.B., polio, influenza, cholera. |
| 3. Deficiency | 3. Inadequate diet | 3. Kwashiorkor, marasmus, scurvy, rickets, anemia, goiter, xerophthalmia, beriberi. |
| 4. Inherited | 4. Defective genes passed on from parents to offspring. | 4. Haemophilia, sickle-cell anemia, cystic fibrosis. |
| 5. Degenerative | 5. Organs and tissues wear away and do not work so well with age. | 5. Arthritis, poor sight, and hearing defects. |
| 6. Mental | 6. Brain damage or psychological. | 6. Depression, paranoia. |
| 7. Social | 7. Social interactions with family friends and strangers. | 7. Drug dependence, alcoholism, smoking, unhealthy lifestyles, and social isolation. |
Extrinsic or External Factors
The disease-causing (pathogenic) external agents which enter the human body from outside are referred to as extrinsic factors.
The important extrinsic factors which upset human health are the following:
- Unbalanced diet or inadequate diet.
- Disease-causing microorganisms such as viruses, bacteria, fungi, protozoans, helminths, worms, etc.
- Environmental pollutants
- Tobacco, alcohol, and narcotic drugs.
Extrinsic factors affect the health of our body by interfering with the normal functioning of the body system.
The most important diseases caused by extrinsic factors are:
- Kwashiorkor
- Marasmus
- Obesity
- Night-blindness
- Beriberi
- Pellagra
- Scurvy
- Anaemia
- Goitre
- Rickets
- Fluorosis
- Food poisoning
- Diarrhea
- Malaria
- AIDS
- Influenza
- Cholera
- Measles
- Chickenpox
- Tuberculosis
- Pneumonia
- Tetanus
- Leprosy
- Poliomyelitis
- Conjunctivitis
- Rabies
- Dengu
- Hepatitis (Jaundice)
- Kala-azar
- Amoebic dysentery
- Giardiasis
- Skin disease (Ringworm)
(i) Disease-causing Microorganisms or Pathogens:
The pathogens include viruses, bacteria, fungi, protozoans, helminths, worms, etc. These pathogens are transferred to the human body through air, contaminated water, food, soil, physical contact, sexual contact, and animals.
(ii) Inadequate diet:
The absence of a nutritional diet makes a person unhealthy. Unhealthy persons are more susceptible to diseases or infections. Deficiency of nutrients in the diet results in a number of deficiency diseases in human beings, e.g., night blindness, beriberi, scurvy, etc. An unbalanced diet may cause obesity.
Some Common Deficiency Diseases of Human beings
| Deficient Nutrient | Disease |
| 1. Protein | Kwashiorkor |
| 2. Protein and total food calories | Marasmus |
| 3. Vitamin A | Xeropthalmia |
| 4. Vitamin A | Night blindness |
| 5. Vitamin B1 | Beriberi |
| 6. Vitamin B2 | Cheilosis |
| 7. Niacin | Pellagra |
| 8. Vitamin B12 | Pernicious Anaemia |
| 9. Iron | Microcytic Anaemia |
| 10. Vitamin C | Scurvy |
| 11. Vitamin D | Rickets (in children) |
| 12. Vitamin K | Bleeding Disease |
| 13. Fluorine | Dental caries |
| 14. Iodine | Goitre |
(iii) Environment Pollutants:
Various environmental pollutants such as gases (e.g., oxides of carbon, oxides of nitrogen, and oxides of sulphur), particulate matter, industrial chemicals, heavy metals, (e.g., mercury, lead, cadmium, and arsenic), pesticides, etc., may contribute to ailments.
(iv) Tobacco, Alcohol, and Narcotic Drugs:
Continuous use of tobacco, alcohol, and narcotic drugs results in harmful effects leading to chronic diseases.
Levels of Immediate Causes
There exist various levels of immediate causes. For example, if a few babies are suffering from loose motions, we can say that an immediate cause of infection or disease is a virus. Such immediate causes of diseases are called first-level causes. At this juncture, we may ask from where this virus comes. The answer would be that the virus came from unclean (contaminated) drinking water. We may think why few babies develop loose motions after drinking unclean water in society.
One reason might be that in comparison to others, these few babies are not healthy and, therefore, are likely to get the disease when exposed to risk. Our study may result in the question of why are these few babies not healthy. The answer would be that perhaps these are underfed (not well nourished). Thus, lack of good nourishment becomes the second-level cause of the disease the babies suffer from. These babies are not properly fed because they belong to the poor household. Therefore, poor public services provide unclean drinking water in the region where such babies are living in society, and poverty becomes the third level cause of the disease.
Possibly, some genetic difference in these few babies might be the reason that makes them more likely to suffer from loose motions when exposed to unclean water containing such a virus. Genetic differences or poor nourishment are contributory causes of the diseases. Contributory causes themselves do not lead to disease.