HIV tends to be the STI that people are most worried about. That's understandable, because HIV leads to AIDS and AIDS is fatal, even though treatment can delay the onset of AIDS for 20 years or more. HIV leads to AIDS when the body's immune system becomes so damaged that infections that a normal healthy person would overcome take hold. When an HIV positive person gets one of these serious infections, they are said to have AIDS.
58,000 people in the UK are thought to have HIV. About a third of HIV positive people don't know they are infected. The UK government estimates that just under a half of all HIV positive adults in the UK are gay men. Government data also shows that heterosexual HIV infection in the UK reflects the fact that global HIV infection is focused in sub-Saharan Africa. In England, Wales and Northern Ireland, of the HIV-infected heterosexual patients seen for care in 2003 and for whom ethnicity was reported, 70% were black-African. In 2005, the government estimates that 7,750 people were diagnosed with HIV with HIV diagnoses among heterosexuals remaining high but relatively stable at around 4400 new cases diagnosed.
In terms of geographical distribution, London has the highest rates of people accessing HIV treatment and services. Elsewhere in England, Greater Manchester, Surrey, Sussex, Bedfordshire, Hertfordshire and the Thames Valley all have relatively high numbers of HIV infection.
As for HIV symptoms, there normally aren't any. That's why a third of HIV positive people don't know they have it.
You catch HIV if certain body fluids from an HIV positive person mix with yours. Typically the body fluids involved in spreading HIV are a man's cum and pre-cum, vaginal secretions and blood. Unprotected vaginal and anal sex tend to be the occasion when these body fluids mix most easily, but the act of having unprotected sex with an HIV positive person doesn't mean you are definitely going to acquire HIV. The reason for this is that the HIV virus has to get into your bloodstream, probably through damaged skin or the absorbent anal lining.
That means any activity or other STI that damages your skin or the lining of your vagina, penis or anus is going to increase your chances of catching HIV from an HIV partner. This is why unprotected anal sex is particularly risky and why having one STI increases your chances of catching others.
All other things being equal, unprotected oral sex is much safer than unprotected vaginal or anal sex, particularly if you don't get a man's cum in your mouth. That said, there are cases of HIV transmission through oral sex. To limit the risks, you need to make sure that you don't have ulcers or cuts in your mouth if you're going down on someone. For that reason, don't floss or brush your teeth right before oral sex.
To test for HIV you need to give a saliva or blood sample. But before going for a test, you need to understand that you may have to wait up to 3 months before going for a test. That's because an HIV test detects HIV antibodies, and won't appear until 3 months after infection occurs. However, if you think you have had unprotected sex with someone who has a very high chance of being HIV positive, you should go to a hospital or doctor within 72 hours of having sex, as there's treatment that can reduce the probability of you contracting HIV.