First thing you should know about this disease is, this is one of the 3 lethal viral diseases which attacks the immune system and is incurable. There is however a vaccine for it and thank god for that, becoz this disease is a frequent killer amongst feral stray populations and outdoor cats.

How does it spread?

The virus is released from the body in saliva and nasal secretions. It is also transmitted, though not as often, in urine, feces, and milk from infected cats.

Therefore cat-to-cat transfer of the virus may occur from a bite wound, during mutual grooming, and sometimes even through the shared use of litter boxes and feeding dishes. Transmission can also take place from an infected mother cat to her kittens, either before they are born or while they are nursing.

FeLV doesn't survive long outside a cat's body, normally less than a few hours under normal household conditions.

What does it do?

It attacks the immune system, to then cause infection, usually in the intestines, and eventually attacks the bone marrow. Eventually the cat succumbs to the amount of infections and tumors. It is possible for the cat’s immune system to fight of the disease if strong enough. There is no known treatment, nor palliative therapy.

How can I tell?

Symptoms vary greatly, so leave this up to your vet. A blood test will tell him more. Lethargy, fever, refusing food due to mouth infection, vomiting, diarrhea…. CAN indicate this, but there is a number of causes with these symptoms. These symptoms should urge you in any case to see a vet.

Feline Leukemia is a deadly disease, so if your cat is an outdoor creature, please, have him vaccinated. You never know if there aren’t any cats around that have this disease. See, the pain in the ass is that these kind of deadly viruses that attack the immune system aren’t lethal immediately. There is a theory in the veterinary world that states that the reason these diseases do not kill immediately is because a cat is by nature rather solitary. With the animal dead, the virus itself would perish. Therefore, cats infected with these sleeper-diseases can live for a long time before they succumb.

Outdoor cats in the highest risk-category are males before castration and females before sterilization btw. Why? Well, males will fight for females and territory, which means wounds and blood, aka the disease transfers via blood. Females in heat will often be bitten by males while they mate, aka more blood transfer.