Zoonotic diseases can be caused by bacteria, viruses, fungi, parasites, or prions. Because organisms are more readily transmitted between closely related hosts, most of these agents are pathogens of mammals. A particularly large number of diseases are shared by people and nonhuman primates. Birds, reptiles, amphibians, fish, and invertebrates can also be sources of infection. Many of the zoonotic agents in poikilotherms are parasitic, but these species can also carry zoonotic bacteria and viruses including Salmonella, West Nile virus, and opportunistic Mycobacterium spp. People are incidental hosts for many zoonoses; however, some agents have both human and animal reservoirs. In some cases, animal reservoirs have been revealed after a disease was controlled in people. Yellow fever, for example, is known to have a zoonotic jungle cycle in nonhuman primates, as well as an urban cycle maintained in people. Wildlife is increasingly recognized as a reservoir for zoonoses, including some that were thought to be strictly livestock pathogens. Reverse zoonoses are caused by human pathogens transmitted to animals. In some cases, these agents can later infect people. For example, Mycobacterium tuberculosis, the agent of human tuberculosis, can colonize the bovine udder and be shed in milk.
The occurrence of a pathogen in both people and animals does not always mean it is a significant zoonosis. Some diseases are acquired from the environment, and transmission between animal or human hosts is either absent, or occurs very rarely and under unusual conditions. These are considered infections common to people and animals rather than true zoonoses. Histoplasmosis, blastomycosis, and coccidioidomycosis, for example, are acquired by inhaling the microconidia of soil fungi, but the organisms exist as yeasts in tissues. Some agents, such as Candida spp, are widespread commensals in healthy people and animals and can cause disease when the host becomes debilitated. Although such organisms might be transmitted from animals to people, this transfer has little epidemiologic significance. In some cases, organisms were once thought to be zoonotic, but current knowledge, particularly the use of genetic techniques, has changed that perception. For example, Streptococcus agalactiae was once thought to be acquired from animals, but most strains in people are now known to be distinct from animal strains. There are also some organisms (eg, simian foamy viruses) that can be transmitted from animals to people, but with no currently known consequences.