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 humans and nonhuman primates. Birds, reptiles, amphibians, fish, and invertebrates can also be sources of infection (see Zoonoses: Global Zoonoses a). 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. Humans 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 humans. Yellow fever, for example, is known to have a zoonotic jungle cycle in nonhuman primates, as well as an urban cycle maintained in humans. 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 that are transmitted to animals. In some cases, these agents can later infect humans. 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 humans and animals does not always mean that it is a significant zoonosis. Some diseases are acquired from the environment and are not transmitted between animal or human hosts. These are considered infections common to humans and animals. Histoplasmosis, for example, is acquired by inhaling the microconidia of soil fungi, but the organism exists as a yeast in tissues. Some agents, such as Candida spp, are widespread commensals in healthy humans and animals and can cause disease when the host becomes debilitated. While such organisms might be transmitted from animals to humans, this transfer has little epidemiologic significance. Also, current knowledge can change rapidly with the use of genetic techniques. For example, Streptococcus agalactiae was once thought to be acquired from animals, but most strains in humans are now known to be distinct from animal strains.

Last full review/revision March 2012 by James A. Roth, DVM, PhD, DACVM; Anna Rovid Spickler, DVM, PhD