It is only possible to transmit HIV through saliva if there are co-factors such as bleeding gums, throat or urethral infections or a high viral load. Saliva does carry the HIV virus but in such low quantities that it is not possible to pass on the infection through kissing or spitting as long as there are no open sores or bleeding gums which result in the exchange of blood. Even cases involving the transference of HIV through saliva with co-factors are extremely rare. However, infection is possible through oral sex but to a much lower degree than anal or vaginal sex.
High concentrations of HIV are present in blood, vaginal fluid, semen, breast milk and any other body fluids which contain blood. Any exchange of these fluids between an infected and a non-infected person is highly risky. There are very low quantities of HIV in saliva so it is not possible to transmit HIV through saliva alone as, to become infected with the virus, there has to be a sufficient quantity of the virus transferred. There is no transmission risk from kissing unless both partners have severely bleeding gums or large open sores in their mouth. There is no risk from sharing glasses, spitting or sneezing as the virus cannot spread or maintain infectiousness in the open air.
Oral sex, however, can be dangerous if one of the partners has the HIV virus albeit to a much lesser extent than penetrative sex. There is the chance of transmitting HIV through saliva if the recipient of the action has the virus and the giver of the action gets infected fluid in their mouth and they have an open sore such as a mouth ulcer or bleeding gums. There is a risk that the infection can enter the bloodstream in this way.
If the performer of oral sex is HIV-infected and has bleeding gums or an open wound in their mouth, then there is also a very small risk of infecting their partner. If none of these co-factors are present, then the risk of transmitting HIV through saliva during oral sex could be said to be non-existent. This cannot be said for other sexually transmitted diseases such as herpes, gonorrhea and syphilis.
Any sexual contact with an HIV-infected person carries some risk whether a lot or little. It is always advisable to use a condom or a latex barrier even when performing or receiving oral sex. Though there is very little risk from kissing, those who carry the virus are advised to avoid deep, open-mouth kissing especially when there is an oral hygiene issue. The risk of transference is remote but it is better to be safe than sorry.