The following points highlight the five main types of disease agents that causes diseases in humans.
1. Biological Infectious Agents:
These are also called pathogens (Gr. pathos means disease; genesis means producing). These are those micro-organisms which when successfully infect the human body, multiply and produce toxins in incubation period which interfere with the normal functioning of the body and cause a disease. These include bacteria, viruses, rickettsias, fungi, protozoans, helminths etc. Out of these, protozoans are one-celled protists, while helminths are multicellular animals.
2. Chemical agents which may be endogenous e.g., urea, uric acid, hormones, enzymes etc.; or exogenous e.g. pollutants like gases, dust, metals, fumes; and allergens like spores, pollens etc.
3. Nutritive agents e.g., minerals, carbohydrates, proteins, fats, vitamins and water.
4. Physical agents e.g., heat (stroke), cold (frost bite), radiations, sound (impaired hearing), etc.
5. Mechanical Agents:
These include injuries, fractures, sprains, dislocations, etc.
Out of these five types of disease agents, biological agents or pathogens; exogenous chemical agents like addictives (e.g., alcohol, tobacco and drugs), pollutants and allergens; nutritive agents; physical agents and mechanical agents are collectively called extrinsic or external factors.
On the other hand, endogenous chemical agents and genetic mutations are collectively called intrinsic or internal factors affecting the human health. The diseases caused by the intrinsic factors are called organic or metabolic diseases and include genetic diseases like haemophilia, sickle-cell anemia; and four types of non-communicable diseases, e.g., heart attack, arthritis, diabetes, haemophilia, allergy, etc.
These five types of disease agents are called primary or immediate causes of the diseases, while the chances of diseases are further increased by certain contributory causes like:
(i) Lack of good nourishment (second-level cause) which decreases the disease-resistance power of an individual.
(ii) Genetic differences between the organisms.
(iii) Poverty and lack of public hygiene services (third-level causes) which increases the chances of diseases.