Feline leukemia virus (FeLV) is second only to trauma as the leading cause of death in cats, killing 85% of persistently infected felines within three years of diagnosis. However, not all cats infected with FeLV progress to persistent infection, 70% of infected felines are able to eliminate the virus on their own. Transmission only occurs between cats, typically through saliva, blood, and to some extent, urine and feces. The virus causes immunosuppression and is a common cause of cancer, several blood disorders, and leaves a cat vulnerable to secondary infections. Diagnosis is by ELISA to detect the presence of FeLV p27, antigen and positive results are usually confirmed by IFA which can also detect the presence of virus particles within white blood cells, usually an indication of more advanced infection.