Feline Leukemia Virus (FeLV) is a retrovirus that destroys cells by replicating the virus within infected cells. It is the most common cause of cancer among cats, and can lead to blood disorders or severe immune deficiencies, making an infected cat susceptible to infections.
The virus can be spread through direct contact with an infected cat, such as through a bite or through mutual
grooming, 
or through contact with the feces, saliva, or milk or an infected cat. The virus does not live long outside an infected cat, and the risk of infection from the virus shed in saliva or excrement usually only lasts a few hours.
Kittens are much more susceptible to FeLV infection than adult cats. As kittens mature, they develop a resistance to the virus. Cats that are at a greatest risk to become infected with FeLV are those cats that live with other cats that are or could be infected, cats that are allowed to roam free outdoors where they may come into contact with infected cats, and kittens that are born to mothers who are infected with the virus. Infected mother cats can infect their kittens either in utero or through their milk.
Cats with FeLV may display a variety of symptoms. For many cats, 
the onset of symptoms is very gradual and may not be obvious until many weeks or even years after the cat had been infected. Common symptoms include the following:
- Loss of appetite
- Poor coat condition
- Persistent diarrhea
- Enlarged lymph nodes
- Progressive weight loss
- Persistent fever
- Skin, upper respiratory tract, and urinary tract infections
- Inflammation of the gums and mouth
- Seizures
- Reproductive failures or spontaneous abortions in un-spayed females
The best form of prevention against FeLV is to keep your cats indoors, where they will not be exposed to other cats that may be infected. Cats may be let outdoors if supervised or kept in an enclosure, where exposure to other cats is eliminated or at least greatly minimized. Before adopting a new cat, have the animal tested for FeLV, to avoid exposing other cats to it. Consider having your cats vaccinated against FeLV, to prevent infection if the cat should get out of the house and be exposed to an infected cat.
