This article is from the Feline Leukemia (Cats) FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
According to the literature, neonatal kittens are 100% susceptible to catching the virus from one exposure. 8-week-old weanlings are 85% susceptible from one exposure. There is serious debate over the likelihood of non-vaccinated, healthy adult cats becoming infected with the virus but it seems that ~40% of cats exposed become immune, ~30 percent become persistently infected (show acute signs of a related-disease) and ~30 percent become infected, but the virus is latent in their system. (Hardy, et al, 1980).