Good questions. I truly don't know the answers - my experience with felines is all of six days worth. I'll ask when I see him again, but in the meantime a Google search is the way to go.
One thing... QUARTINE IMMEDIATELY! If your cat has come in contact at all with this sick cat... and that includes petting both cats without washing hands in between... IMMEDIATELY! Take your cat to the vet!
If this cat does already have Feline Leukemia and it's not a false positive, your cat has a very high chance of being infected. I don't mean to be alarmist but my cats both have (very mild) feline herpes. The cause? Jabez interacted with a friend's formerly-stray cat for 3 minutes! So separate them IMMEDIATELY, far away from each other. No shared food, no shared water, and no shared air if possible--and wash your hands before handling your current cat!