Feline Leukemia Virus:
Feline Leukemia Virus (FeLV) is a virus that suppresses the immune system. It only affects cats, not humans or other animals. It is passed through saliva, blood and sometimes milk of mothering cats, urine and feces. Grooming and fighting seem to be the most common ways for it to spread. Keeping your cat indoors only minimizes the risk for contracting this disease considerably. Diagnosis is a simple blood test by your vet. It is recommended that if a cat tests positive for this disease to have it periodically re-checked as sometimes cats give a false negative reaction.
Cats can be treated with this disease, though there is currently no cure. Twice yearly physical exams are recommended to prevent/treat any issues that may cause complications. If kept in otherwise good health, cats with this disease can live symptom free lives as this disease takes time to slow things down. Cats can live normal lives for prolonged periods of time. Once diagnosed with FeLV, it’s important to monitor weight, appetite, activity level, elimination habits, appearance of mouth and eyes and behavior. The sooner you can relay any concerns to your vet, the sooner they can treat the specific issue to keep it from affecting the cat’s immune system. Cats with FeLV may have a normal lifespan if other illnesses can be prevented.
Infected cats should be kept indoors only and away from uninfected cats to prevent further spread.
Information extracted from:
https://pets.webmd.com/cats/facts-about-feline-leukemia-virus#1
https://www.petmd.com/cat/conditions/infectious-parasitic/c_ct_feline_leukemia?page=2