Zoonoses are diseases and infections of animal origin that are communicable to man. The other species are invariably other vertebrate species. The infectious agents cross the "species barrier", a poorly understood explanation for why humans are protected from catching the diseases of higher animals. Zoonotic agents may be viral, bacterial, parasitic or fungal. The frequency of zoonotic infection varies with the type and source, as well as with the geographical location and occupational exposure. In the majority of zoonoses the infection remains limited to the affected individual, and person-to-person transmission is rare or exceptional. Often, the infection causes observable disease only in man; the animal 'carrier' being symptomless or only mildly sick, as is the case, for example, in Q-fever.
There are over 150 zoonoses carried by a wide variety of animals. The highest incidence of zoonoses is noticed in persons who come in close contact with animals or animal products or those who share with animals environments containing suitable vectors. Common zoonoses are anthrax, brucellosis, Chaga's disease, equine encephalomyelitis, equine infectious anaemia, foot-and-mouth disease, glanders, hydatid disease, leptospirosis, listeriosis, liver flukes, louping illness, lymphocytic choriomeningitis, Newcastle disease, psittacosis-ornithosis, Q fever, rabies, rat-bite fever, rift valley fever, ringworm, Rocky Mountain disease, Russian spring-summer virus, salmonellosis, scabies, schistosomiasis influenza, echinococcosis tapeworms, tick-bite fever, tick paralysis, toxocara, toxoplasmosis, trichinosis, tuberculosis, tularaemia, vesicular stomatitis, Warburg disease, Wesselsbron disease, typhoid, yellow fever, and plague. In spite of their varied nature these diseases have one common feature - in nature, they are transmissable from animals to man either directly or through animal products and sometimes through invertebrate vectors (insects, ticks, mites, molluscs). With the AIDS epidemic there has been an increase in the number of people with suppressed immune systems, a condition which raises susceptibility to all kinds of diseases, including the zoonoses. In developed countries where zoonoses are relatively rare, domestic animals and pets are a more common source of infection.