All that has already been said on this subject is true - it's not a death sentence, it's species specific and the only KNOWN source of infection is through bite wounds.
As Spencer said food and water bowls should be treated as tough they could be infective and flea control should be a priority.

A few things that may help your cat include the obvious - prompt veterinary treatment for any secondary infections. Treatment may take longer to show results.
FIV cats should not be given raw meat as there is a greater risk of Toxoplasma gondii infection.
Evening Primrose oil may give some increases in blood counts and body weight ( one 55pmg capsule each day).

The mother of your kitten should be tested again as this seems the most likely source of your kittens infection. The vet is asking for retesting because if the source is the mother then it may be the antibodies she passed on to the kitten that are showing positive in the test rather than your kittens own infection. Although, if your kitten is over six months this is highly unlikely.
Other than this the only source of infection has to be a stray cat entering the house where the kitten was at some stage. Or possibly food or water contaminated from an infected cat.