Feline leukemia virus (FeLV) is one of the most important infectious pathogens that causes death in cats and is broadly spread worldwide. FelV was discovered among cats that lived in a cluster-household where several animals had developed lymphosarcoma. The virus is an enveloped, positive single-stranded RNA retrovirus. Cats are usually infected by direct contact with infected cats, mostly via oronasal exposure to saliva and nasal secretions containing high levels of the virus especially through mutual grooming and sharing food dishes or water bowls. 

Feline immunodeficiency virus (FIV) is a lentivirus that affects cats worldwide. A vaccine is available although its efficacy remains uncertain. Cats will test positive for FIV antibodies after vaccination. The transmission is via deep bite wounds, where the infected cat's saliva enters the other cat's tissues.