Feline Leukemia Virus (FeLV)
FeLV is a cancer causing virus that is transmitted from cat the cat through fighting or meeting. FeLV cannot survive for long outside of a cats body and is easily destroyed with the most disinfectants and detergents.
Feline immunodeficiency virus (FIV)
FIV suppresses a cats immune system. It is spread through deep bite wounds. Because sterilized cats fight much less than intact cats, neutered FIV positive cats generally live safely with uninfected cats both in and outdoors. But, while the risk of transmitting the virus to another cat is very low, it is not nonexistent.
The American Association of feline practitioners recommends against routine euthanasia of healthy FeLV and FIV positive cats!