What are the chances that my cat has FIV (feline HIV) or feline leukemia?

I just adopted a cat who used to be a stray, although she has lived indoors in a home for the past year. Before that, she lived with two outdoor cats. She seems healthy and eats well, etc. The vet checked her over on Friday and said he thinks she's fine. He's only ordering bloodwork because he said he wants to be on the safe side to protect my two current indoor cats and because of the new cat's questionable past. I'm taking her for the bloodwork tonight, but just wondering if anyone out there knows about the diseases and whether they think I'm worrying for nothing!! Signed, Worried Mom :-)