When we do this exercise, we realise that health and disease in human communities are very complex issues, with many interconnected causes. We also realise that the ideas of what 'health' and 'disease' mean are themselves very complicated. When we ask what causes diseases and how we prevent them, we have to begin by asking what these notions mean. Health is a state of physical, mental and social well-being. The health of an individual is dependent on his/her physical surroundings and his/her economic status. Diseases are classified as acute or chronic, depending on their duration. Disease may be due to infectious or noninfectious causes. Infectious agents belong to different categories of organisms and may be unicellular and microscopic or multicellular. The category to which a disease-causing organism belongs decides the type of treatment. Infectious agents are spread through air, water, physical contact or vectors. Prevention of disease is more desirable than its successful treatment. Infectious diseases can be prevented by public health hygiene measures that reduce exposure to infectious agents. Infectious diseases can also be prevented by using immunisation. Effective prevention of infectious diseases in the community requires that everyone should have access to public hygiene and immunisation. Traditional Indian and Chinese medicinal systems sometimes deliberately rubbed the skin crusts from smallpox victims into the skin of healthy people. They thus hoped to induce a mild form of smallpox that would create resistance against the disease. Famously, two centuries ago, an English physician named Edward Jenner, realised that milkmaids who had had cowpox did not catch smallpox even during epidemics. Cowpox is a very mild disease. Jenner tried deliberately giving cowpox to people, and found that they were now resistant to smallpox. This was because the smallpox virus is closely related to the cowpox virus. 'Cow' is 'vacca' in Latin, and cowpox is 'vaccinia'. From these roots, the word 'vaccination' has come into our usage.