Feline leukemia is a cancerous disease caused by feline leukemia virus (FeLV). FeLV causes diseases other than leukemia including other cancers and immunodeficiency. Cats may not start to show signs of disease for months or years after being infected with FeLV. Infection with FeLV is a major cause of illness and death in domestic cats. Approximately 2.3% of cats in the United States are infected with FeLV. Read more at peteducation.com and bestfriends.org.