Salivary transmission of HIV is relatively rare. Saliva is thought to protect against HIV-transmitting white blood cells because of its unique low salt concentration, which helps to kill these cells. However, HIV is successfully transmitted via seminal fluid, breast milk and colostrum( white cell-rich fluid secreted in the first days of breastfeeding).

Under normal circumstances, there is just about one-fifth of a teaspoon of saliva in the mouth. Deposited semen typically would equal more than four times that volume and mother's milk much, much more. So it's easy to see that saliva can't kill the infected cells because there's just too much of the saltier substances.

These findings have implications for the actual cause of HIV infection. There is very good evidence that infected white blood cells are the main transmitting agent of HIV in the vagina and the rectum.

While many physicians still believe that HIV is transmitted by free virus from HIV-infected individuals, most cell-free virus is not infectious, probably because HIV carriers make sufficient antibodies, which bind to the virus and hamper its ability to infect other cells.

Conversely, HIV-positive milk and semen are highly infectious, probably because these fluids contain white blood cells that are infected by HIV and the antibodies can't get inside these cells to attack the virus.

There is effort going on to develop anti-HIV gel that can mimic the protective effects of saliva in the vagina and rectum, environments that are both more salty than the mouth.

SOURCE: The Journal of Infectious Diseases 2000;181.