The only other cat you should bring into your home is one that has
already tested positive for FIV. If you bring in another cat that isn't
FIV+, chances are exceedingly high it will get infected. That you don't
want. But another FIV+ cat already has the virus, and so neither will be
a danger to the other in that regard.

Check with the nearest no-kill shelter. They often keep for adoption
FIV+ and FeLV+ cats, rather than destroying them.

Q: Does Jimmy genuinely have the disease, or has he just tested positive
for the virus?

First off- How old is your cat?  Cats less than 6-8 months old can test
positive due to maternally-derived antibodies without actually being
infected.  If your cat is younger than 6 months, retest in a month or two.
It takes about six months for maternally-derived FIV antibodies to
dissapate.

Second- If your cat is older than 6-8 months and your vet used an in-house
test that he performed himself (FIV-ELISA Snap Test) you can have your cat's
blood retested with a labortory test called the Western Blot (immunoblot).
This test detects more specific antibodies. The in-house FIV-ELISA Snap
test, and to a lesser degree, the Westernblot,  produce a lot of false
positive results because both test for antibodies to the virus and not the
antigen or virus itself.   False-positives are always more common when
testing for antibodies.

Third- If your cat was vacinated against FIV he will test positive for FIV
on both, the FIV-ELISA Snap Test and the Western Blot.  The only test that
can differentiate FIV-vaccinated cats from FIV-infected cats is the FIV-DNA
PCR assay offered by UC-Davis.
The PCR DNA assay detects the viral genome directly and not antibodies, thus
its the most specific and the most sensitive test available for FIV.

If I were in your situation, I'd opt for the PCR DNA assay because it can
also distinguish false positive results due to maternally-derived antibodies
from infected cats without having to wait 6-8 months for antibodies to
dissapate.

FIV is not nearly as contagious or as easily transmitted as many people
think. It usually takes a deep bite wound to transmit enough virus directly
into the body to cause infection. As long as the cats are neutered and don't
fight, its relatively safe to keep an FIV-positive cat in a household with
other cats that are FIV-negative.  I know of many mixed households where
FIV+ and FIV- cats have lived together for many years without the FIV- cats
ever becoming positive.

Before you make any decisions, have Jimmy retested with the FIV DNA PCR
assay.  He may not even be infected!

Best of luck,

Phil