Feline immunodeficiency (FIV-Feline Immunodeficiency Virus) and feline leukemia (FeLV-Feline Leukemia Virus) are common feline viral diseases caused by Retroviridae viruses. Although they are related, FIV and FeLV are different structurally and genetically. Both viruses attack the immune system leaving the cat vulnerable to many other infections. They are species-specific and cause exclusively feline infections in cats and feral cats (species Felidae), so it is necessary to emphasize that there is no possibility of transferring the infection to humans. Infection is most commonly transferred horizontally by bite, via saliva. A vertical transfer is present but rare. Stray cats have a predisposition for contracting FIV, as well as noncastrated tomcats, and cats whose organism is already weakened by some other disease.