12th Class Biology Microbes in Human Welfare Disease Agents

The disease agent is a factor (substance or force) which causes a disease by its excess or deficiency or absence. These agents are of five main types :

(1) Biological Agents : These include viruses, rickettsias, bacteria, fungi, protozoans, helminthes and arthropods. The biological agents are called pathogens (Gr. Pathos = disease; genes = producing).

(2) Nutrient Agents : These comprise food components such as carbohydrates, fats, proteins, minerals, vitamins and water.

(3) Chemical Agents : These are further of two types

(i) Endogenous Chemical Agents : These are formed in the body itself and include hormones, enzymes, urea and uric acid.

(ii) Exogenous Chemical Agents : These enter the body from outside by inhalation, ingestion or inoculation. Pollutants (fumes, gases, dusts, metals) and allergens (spores, pollen) are examples.

(4) Physical Agents : These include heat, cold, humidity, pressure radiation, electricity and sound.

(5) Mechanical Agents : These comprise chronic friction or other mechanical forces which result in injury, sprain, dislocation fracture.

(6) Deficiency and Excess of substances : e.g. Hormones, enzymes.

Some diseases are caused by genetic disorders and lack or underdevelopment of organs. The agents for certain diseases such as peptic ulcers, coronary heart diseases and hypertension, are not fully known.