A zoonosis is any animal disease that can be spread to humans. Zoonosis diseases are transmitted either directly by germs or by parasitic organisms that travel between infected animals and humans.
Cat-scratch fever and toxoplasmosis are examples of zoonosis that are spread directly from animals to humans. Cat-scratch fever is a viral infection transmitted by the scratch or bite of a cat, characterized by fever and swelling of the lymph glands. Toxoplasmosis is an infection caused by a protozoan parasite—transmitted by dogs, cats, and other animals—that, in humans, invades tissues and sometimes damages the central nervous system. By the same token, people can become infected with rabies if bitten by a dog or other animal.
Lyme disease is a disease transmitted by ticks that affect several of the body's systems. Rocky Mountain spotted fever is a sometimes-fatal bacterial infection transmitted by a ticks. Both are examples of the type of zoonosis that are indirectly spread to humans by parasitic organisms—in this case, ticks. Lyme disease is initially characterized by a rash followed by flulike symptoms including fever, joint pain, and headache. If left untreated lyme disease can lead to chronic arthritis or nerve and heart disorders.
Most zoonoses (plural form of zoonosis) are relatively rare and can be treated once detected. Such sensible actions as regularly vaccinating pets and wearing long-sleeved shirts and pants when hiking can prevent the spread of most zoonoses.
Sources: The American Medical Association Encyclopedia of Medicine, p. 1088; Current Health 2, 27, no. 7 (March 1991), pp. 11-13.