FIV (feline immunodeficiency virus) works in cats the way HIV works in humans. You cannot get sick from a cat with FIV. It's very hard for an FIV+ cat to give it to another cat. The risk factors are very similar to humans: blood to blood contact or mating. For these reasons FIV+ cats should be solo or only with other FIV+ cats unless all cats involved are known not to ever fight.