Most developing countries in Africa and Asia are located in the tropics (23° north and south of the equator). The poor productivity of ruminants in these countries is mostly due to inadequate nutrition, infectious diseases, low rates of reproduction, poor genotype, high ambient temperatures and socioeconomic constraints. In addition to infectious diseases which have long since been eradicated or brought under control in developed countries, there are diseases unique to the tropical environment. In the past, the diagnosis of most diseases was based on serological tests. The enzyme-linked immunosorbent assay (ELISA), a rapid method of identifying viral and bacterial infections, is contributing to the eradication or control of most epidemic diseases which inflict high mortality Less attention has, however, been given to subclinical diseases which adversely affect reproduction, growth rates and reduced performance (draught power, milk and meat). The interaction of veterinary epidemiology and agricultural economies has helped epidemiologists and veterinarians to focus on the economic importance of animal diseases. The emerging trends in animal health include new diagnostic methods using DNA probes, new vaccines, sustainable parasite control schemes and herd health monitoring. The study of infectious diseases of veterinary importance will continue to be supported by modern techniques of molecular biology. Veterinary curricula should emphasise both animal health and production in the context of current national development occurring in the livestock sector.