"Emerging zoonoses" are defined as zoonotic diseases caused either by apparently new agents, or by previously known microorganisms, appearing in places or in species in which the disease was previously unknown. New animal diseases with an unknown host spectrum are also included in this definition. Natural animal reservoirs represent a more frequent source of new agents of human disease than the sudden appearance of a completely new agent. Factors explaining the emergence of a zoonotic or potentially zoonotic disease are usually complex, involving mechanisms at the molecular level, such as genetic drift and shift, and modification of the immunological status of individuals and populations. Social and ecological conditions influencing population growth and movement, food habits, the environment and many other factors may play a more important role than changes at the molecular level. Diseases associated with changing farming practices, trade and consumer habits. Bacterial enteric diseases due to Salmonella enteritidis and Echerichia coli 0:157 are examples of diseases associated with changing farming practices and consumer habits. The increasing trade in live animals for animal production and research led to the introduction of the New World screwworm to the Libyan Arab Jamahiriya in 1989 and an Ebola-like virus in monkeys in quarantine facilities in the United States of America. The development of the epidemics of bovine spongiform encephalopathy (BSE) in the United Kingdom is due to multiple factors including the increasing use of ruminant proteins as feed for animals. Diseases associated with changing environmental conditions which influence reservoirs, vectors and/or victim species population parameters.(ABSTRACT TRUNCATED AT 250 WORDS)