Fingers crossed for them, and it might be worth having FIV/FeLV tests done. There is an FIV vaccine in AMerica, but it is slightly unreliable at the moment, due to the tests not being able to pick up the difference between the vaccine and the virus, so any vaccinated cat tested would show as positive - not what we want in this country with certain rescue's attitudes towards FIV+ cats.
CC, I wouldnt dismiss what your vet said about your cat, nasty abscesses probably could kill, I had a cat with an abscess, and as it only looked like a small dark mark, my neighbour dismissed my concerns, and it was only when the cat started having smelly gunk pouring out of it that my neighbour realised she was wrong and she was booked in at the vets, but she was really bad that night, and did end up dying 4 days later - vet said it wasn't related, but as she was perfectly healthy before that, I can't quite believe there wasn't a connection. Hers was so bad she had to have drains put in, as well as antibiotics.