Diseases due to infection are prevalent throughout the world. This is especially the case in tropical countries where insect-borne diseases are common and in underdeveloped areas where standards of hygiene and nutrition are low. Almost half of all deaths in developing countries occur in children under the age of 5 from a combination of infection and malnutrition. The main groups of organisms involved are metazoa, protozoa, fungi, bacteria, mycoplasmas, rickettsiae and the viruses. Infections due to metazoa and protozoa occur mainly in tropical areas; thus helminthic infections, malaria, amoebic dysentery, sleeping sickness and leishmaniasis are common diseases in tropical Africa, Central and South America and large parts of Asia but are relatively rare elsewhere. Fungi which cause ringworm of the skin and Candida which cause thrush infections are Blastomyces, Histoplasma and Coccidioides are greatly restricted in their geographical incidence.