What is disease agent ? List any four types of disease agent.

Explanation

Definition. Any substance which causes a disease by its excess or deficiency or absence, is called a disease agent.
Types of disease agents. Disease agents are of five types :
1. Biological (infectious) agents. These are also called pathogens. 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),
5. Mechanical agents. These include injuries, fractures, sprains, dislocations, etc.