Agent The disease agent is a substance , living or non living, or a force, tangible or intangible, the excessive presence or relative lack of which may initiate or perpetuate a disease process. Disease may have a single or a number of independent agents or a complex of two or more factors. Biological agents– living agent of disease Bacteria, Viruses, protozoa, fungi, rickettsia Nutrient agents– Protein, carbohydrate, vitamins, minerals and water Any excess or deficiency of the nutrient elements may result in nutritional disorders, anemia, goiter, obesity Physical agents – Exposure to excessive heat, cold, humidity, pressure, radiation, sound Chemical agents– metal fumes, dust, gases, insecticides Mechanical agents – exposure to chronic friction and other mechanical forces may result in crushing, tearing, sprains, dislocations Social agents – poverty, smoking, abuse of drugs, unhealthy life style, social isolation