What you have to remember is that the HIV virus is spread via bodily fluids. Kissing that does not involve exchange of fluids is not risky (closed mouth or "social" kissing). Kissing that involves the exchange of fluids, on the other hand, poses a small risk if one of the individuals involved has HIV.
The CDC has cautioned against open-mouthed kissing if individuals are HIV positive. Although the risk of transmission is very small, it cannot be ruled out completely.
Answer 2
It is highly unlikely that you will get HIV from open mouthed kissing.. The only reason why the CDC cautions against it, is in cases where both people have open cuts on their lips, or active herpes lesions. It would take more than two cups of saliva to be transferred from one person to another, to get the infection through the saliva, so this method is yes, highly unlikely, but just don't go to any Guiness Book of World Records kissing festivals with a person who you know is infected. =) You also cannot get HIV from oral sex, either, as long as there is no open sore on the mouth of the infected person giving the oral sex.
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Almost impossible to get HIV from kissing; only if the person with HIV had blood in their mouth (say from bleeding gums) and you were deep french kissing.
You can't get infected from a casual kiss.
You can't get a HIV infection from casual kissing. It is not likely to get HIV infected from french kissing unless there is blood in the mouth which could cause the transfer to occur.