The difficulty in coping with communicable diseases lies in the diversity of diseases likely to pose problems in emergencies, as well as in the interaction of risk factors. Tropical-medicine textbooks tend to classify communicable diseases according to the pathogenic agents that cause them. Health education is not limited to the problem of communicable diseases. However, communicable diseases are a useful lead-in for health education. The community must understand well the risks involved in communicable diseases, and participate in controlling them. The incidence of most communicable diseases is seasonal. For example, epidemic meningitis develops during the dry season; the incidence of malaria, diarrhoeal diseases, and respiratory infections increases in the rainy season; and measles epidemics spread more easily during the dry season. It should be noted, however, that the different seasons sometimes correspond to occasions when large numbers of people are gathered together.