Diseases that are transmitted from animals to humans are called zoonoses or zoonotic diseases. They are difficult to control because the existence of an animal reservoir makes it hard to eliminated them. Consider, for example, the problems involved in controlling rodents in the deserts of the southwestern United States that are infected with the plague, or larger herds of wildebeest that carry sleeping sickness in Central Africa. Animal related diseases have caused untold damage to people in the past and continue to be of enormous concern, especially in the tropical areas of the world.

Zoonoses are defined as those infectious diseases that are naturally transmitted between vertebrate animals and humans. The word is derived from the Greek, “zoon”, meaning animal, and “nososs”, meaning disease. The greatest risk of acquiring zoonoses are of course those who work in close proximity to animals, like farmers, veterinarians, slaughterhouse workers, and animal researchers. The most common sources of zoonotic diseases are domestic animals, namely pets and farm animals. Over 100 million cats and dogs are kept as pets in the United States, and although they are rare, more than 30 human diseases can be acquired from them.

With the international movement of animals, importation of zoonotic diseases from one geographical locale to another takes place with increasing frequency. For example, Marburg virus disease was first documented in laboratory workers in Germany after they had contact with monkeys imported from Africa for use in vaccine research.