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.