Zoonoses (singular, zoonosis) are infectious diseases that can be transmitted from wild or domestic animals to humans. Some zoonoses can also be transmitted from humans to other animals. The disease agents that can cause zoonoses include prions (abnormal proteins), bacteria, viruses, fungi, and parasites.
A wide variety of animals can serve as vectors, or carriers, of zoonoses. Wild animals and insects that can spread zoonoses include bats, birds, chimpanzees, fleas, flies, mosquitoes, mice and rats, monkeys, opossums, rabbits, snails, and ticks. Domestic and farm animals that can transmit zoo-noses include cats, dogs, cattle, goats, horses, pigs, and sheep. Animals can transmit zoonoses through biting humans (plague, Lyme disease, rabies), leaving droppings that contain the disease organism (hantavirus infection, toxoplasmosis), or harboring the organism in their tissues or hair (anthrax).
Zoonoses range from mild diseases like cat scratch fever and cowpox to highly fatal illnesses like Ebola and Marburg hemorrhagic fevers. Some zoonoses, like rabies, appear as isolated cases in humans, while others, like AIDS and plague, can spread directly from one human to another and cause worldwide epidemics. Some doctors think that many diseases that are now spread directly among humans, like measles, tuberculosis, influenza, and the common cold, may have started out as zoonoses thousands of years ago.