What is feline leukemia virus? Feline leukemia is a viral disease seen only in cats. It is not transmissible to humans (even children) or to any other species, such as dogs. FeLV is caused by a type of virus, called a retrovirus, which infects the cat’s cells and reprograms them to continually produce copies of the virus. This produces a condition in the cat called persistent viremia, which makes the disease impossible to cure. What happens to a cat who has contracted the virus? The virus adversely affects the cat’s bone marrow and immune system. An infected cat becomes anemic and is unable to fight off even routine infections. Cats with feline leukemia are commonly jaundiced and lethargic, and they experience weight loss, enlarged lymph nodes, and poor stamina. After exposure to the feline leukemia virus, some cats also develop a type of tumor called lymphosarcoma, a type of cancer than can affect many different organs, beginning with the lymph nodes and possibly spreading to the intestines, liver, spleen and brain.