The answer is relatively straightforward although the best book on cat health1 doesn’t provide a definitive and confident answer. There is some uncertainty on transmission. And that also applies to the Cornell website.
The FIV virus is shed in saliva.
Therefore Cat bites are a source of transmission of the virus which causes AIDS (aka FIV). Note: “AIDS” is a layman’s term for FIV which stands for Feline Immunodeficiency Virus Infection. The bite delivers the saliva carrying the virus and deposits it under the skin a bit like a hypodermic needle delivering a vaccination.
Bites are more likely to occur in outside, unneutered cats who fight over territory, which is why at the date of this post, there are several articles in the British online news media about a potential cat AIDS epidemic because there are an estimated 880,000 cats in the UK who are not neutered.
I have no idea how they arrived at that figure. Also, although a free roaming unneutered cat will be at a greater risk of getting FIV, it does not mean he’ll get it. It is a heightened risk factor. The reason for this statement is that tom cats fight more. That said male cats who have been neutered also fight.
USA statistics tell us that in the general cat population the incidence of feline AIDS is 2-4 percent while that figure rises to 3-5 percent for outdoor cats. We can see therefore that the greater risk of an unneutered cat getting feline AIDS is not that huge. The big risk is with outdoor male cats coming into conflict over territory.
A second possible way (on rare occasions) of transmission is when an infected mother passes the virus to her unborn kittens.
Contact between cats appears to be an inefficient route for transmission. It must be a potential route according to Cornell but they appear to be uncertain as mentioned. Cats in multi-cat homes should be tested for FIV.
Also, at a much lower level of risk, it would seem that allogrooming (mutual grooming between cat associates) could at least potentially or very rarely transmit the disease from one to the other because a cat’s saliva is deposited on another cat where it could be licked off by that cat. This is based on what I have read from reliable sources. I sense that the medical profession are not completely clear on this subject.
To return to the scaremongering news articles about an epidemic of feline AIDS. These stories are slightly ignorant or worse. It is sensible to highlight the risk to free roaming unneutered male cats but to overdo it simply makes people unjustifiably nervous about FIV (perhaps that is a good thing on second thoughts). Obviously male cats should be neutered and the major reason for it is to stop them creating more cats as we have enough already, something we are constantly reminded of.
Note 1: Cat Owner’s Home Veterinary Handbook pages 93-94 (third edition)