Feline Leukemia VIRUS is NOT cancer. It is a virus that attacks the cat's immune system and makes them susceptible to many other sicknesses, and causes other problems-like teeth and gum probs, among other things. Do a google search and look up feline leukemia virus.
This is very, very different than the human leukemia.
Leukemia is a cancer is not contagious. At least with humans. I guess a little more research on how it works with cats should be in order.