(i) Biological agents like bacteria, viruses, fungi, protozoa, worms and insects.
(ii) Nutrient agents like carbohydrates, proteins, fats, vitamins, minerals and water. An excess or deficiency of these nutrients may cause nutritional diseases.
(iii) Physical agents like extremes of cold and heat, X-ray, y-rays, pressure, electricity etc.
(iv) Chemical agents like carbon monoxide, pesticides, fertilizers, fumes, dust, gases etc. may cause illness by inhalation, ingestion or direct contact.
(v) Mechanical agents like motor vehicles; machinery etc. may cause injuries and fractures.
Man acts as a host to a number of pathogenic micro-organisms. These micro-organisms attack the host when the immunity (i.e. the defence mechanism) is lost. The host related factors include age, sex, heredity, nutrition, occupation, customs and habits etc.
Certain diseases like measles etc. are more common in childhood, cancer in middle age and tuberculosis in old age. Females are more affected from cancer of various organs than males. Cases of heart attacks are more in males than in females. Essential hypertension, diabetes, mental diseases are due to genetic factors.
Deficiency of proteins may cause kwashiorkor and other problems. Over-eating may lead to obesity and diabetes. Certain habits like smoking may cause lung cancer; open air defecation may cause soil and water pollution which ultimately causes various types of intestinal disorders.
Environment plays a great role for the upkeep of health. A healthy environment is crucial for the health and wellbeing of individuals and communities. The environment may be favourable to host or to agent. If the environment is favourable to agent it will cause disease.
Climatic and seasonal factors may determine whether it will be suitable to a particular disease or not e.g. malaria is more common in rainy season whereas common cold is more common in winter.
Sanitary conditions, provision of potable water supply, education and standard of living as well as biological environment e.g. animals, insects, rodents etc. are important factors in determining incidence of diseases.