Feline Leukemia Virus is a virus that is specific to cats only. It is considered to be the most common cause of serious illness and death in domestic cats. It causes a breakdown in your cat's immune system causing your cat to become susceptible to many diseases which it might otherwise be able to fight off. It CANNOT be transmitted to humans (including children) nor can it be transmitted to other species such as dogs. The National Cancer Institute and the National Institutes of Health have written a CancerNet Factsheet which is available via gopher. See the References section for more information on this.