No one has ever caught HIV from kissing, no matter how hard they went for it, so there's no need to worry about getting passionate and smoochy. Although very small quantities of HIV have been detected in saliva (which makes it possible for some HIV testing kits to work from a saliva swab), your spit also has properties that disable HIV so it would be impossible to swallow enough saliva to become infected with HIV. And anyway, when people talk about the potential risk of catching HIV from saliva they say that you'd have to drink pints of it, which would be extremely hard to do (see how long it takes you to fill even a small glass if you don't believe us) - and pretty revolting too. You can pick up herpes and glandular fever from kissing though, so it's probably best to avoid snogging anyone with visible cold sores around their mouth.