Q: I recently read information on your website that states that humans can "almost certainly not" contract FIV from a cat and that there was no evidence in owners, researchers, etc. However, were any of these people bitten or scratched by an infected cat? Also, can a human contract it by cleaning an infected cat's litter box? What if a cat licks you? "Almost certainly not" is not 100%. I don't have a cat, but my HUGE concern is that my babysitter, who I just hired for my 23-month-old child, has a cat with FIV (and I had never heard of it until two days ago when she told me). Please help!

A: There are a number of reasons for putting your mind at ease, a few I've which I'll mention here. First, feline immunodeficiency virus (FIV) is quite limited in the ways it can be transmitted to cats, the only animals the virus is known to infect. With few exceptions, the bite of an infected cat is required; that is, virus present in the saliva of an infected cat must be violently passed through the skin of another cat. (On rare occasions, the offspring of an infected mother cat may become infected, but obviously this doesn't apply to your specific concerns.)

Researchers have found that some laboratory strains of FIV can pass through mucus membranes (like those lining the oral cavity), perhaps putting cats that live peacefully with an infected cat at risk of infection. One can easily imagine that if an infected cat grooms another, the virus in his saliva could wind up in the mouth of his cat buddy. However, there's only scant evidence that FIV can be transmitted to cats by non-aggressive-that is, non-biting-behavior.

Second, FIV remains viable for only a short period of time outside the cat, so none could be tracked in on your sitter or her clothing. It's widely believed that sharing litter boxes, feeding dishes or living space with FIV-positive cats will not place other resident cats at risk. People who clean litter boxes or feeding dishes used by infected cats are not at risk either.

Why Not 100% Certain?
It's risky to say something like, "humans can never, ever become infected with FIV," because no one will ever conduct a study in which every person on earth is injected with a dose of FIV and then checked later to see if somebody got infected. So we're left with other ways to determine whether the virus poses any risk to humans. One of the ways is to study people with a high risk of exposure, like veterinarians and lab personnel who work directly with the virus. At least one such study has been conducted (and I was one of the guinea pigs), and no evidence of infection was found in anyone, even those who'd been bitten by FIV infected cats or accidentally injected with virus.

There is absolutely no evidence that any person has ever been infected with FIV. Having said this, though, if FIV was to pose a problem, the people most likely at risk would be those most at risk of any kind of infection: the immunosuppressed (like people who have AIDS or who are taking immunosuppressive drugs for a medical condition), the very old, and the very young.

For this reason - and just to be on the safe side - I suggest that people who fit into any of these categories avoid close contact with FIV infected cats (like kissing on the nose or mouth), wash any areas of their body licked by the kitty, and avoid being bitten. These precautions are not based on a deep fear that FIV will infect the person; rather, they're just common sense.