There are two main types of Human Immunodeficiency Virus (HIV): HIV-1 and HIV-2. These are retroviruses and possess an enzyme (reverse transcriptase) that allows them to copy their genetic material and incorporate it into the DNA of a human cell where it lies 'hidden' and protected from immune attack.

HIV selectively invades the immune cell known as a T-helper lymphocyte (also known as T4 or CD4 cells) in which it multiples and, in some cases, destroys the cells or stops them working properly. T4 cells are essential for regulating the activity of other immune cells, and are especially important for triggering the production of antibodies from B lymphocytes.

How HIV is transmitted?
Through contact with infected body fluids such as blood, urine, semen, breast milk and saliva.

HIV may be spread in a number of ways including:

• unprotected sexual contact with a carrier
• from mother to baby in the womb or during birth
• receipt of infected blood via transfusion - blood in the western world is routinely screened but transfusions further abroad may be a route of transmission
• drug abusers sharing needles
• needle stick injury
• collision injury with a carrier during contact sport
• assault (e.g. human bite)
• contact with contaminated items such as toothbrush, shaver
• wet kissing with exchange of saliva, although this risk is thought to be low

Infection is NOT passed on through normal day-to-day activities such as hugging, dry kissing, shaking hands or sharing cutlery or cups.

What HIV symptoms will you notice?
A few weeks after infection with HIV, some people develop a flu-like illness lasting from one to three weeks. This is usually dismissed as a non-specific 'virus'. Symptoms include fever, headache, joint and muscle pains, lethargy, sore throat, mouth ulcers, swollen glands and sometimes a pink rash.

If blood tests are taken at this stage, they may show low levels of T-helper cells and other abnormal findings such as raised liver enzymes. This illness is usually associated with the production of anti-HIV antibodies (i.e. you become HIV-antibody positive - commonly known as HIV positive).

Most people who are HIV positive remain healthy for many years, but are still highly infectious. Others develop vague symptoms such as slight swelling of lymph glands, weight loss, night sweats or unexplained diarrhoea. These symptoms are known as AIDS-related complex (ARC).

Most people who are HIV positive are unaware of the fact and may therefore pass on the infection to others.

AIDS

After a number of years, HIV infection may become increasingly active so that larger numbers of C4 cells are destroyed, and the ability to fight off common infections is reduced. As a result, minor infections that do not trouble healthy individuals can cause serious illnesses (e.g. Pneumocystis and Cryptococcus fungal infections). This stage of the illness is known as acquired immunodeficiency syndrome (AIDS).

What AIDS symptoms will you notice?

• Recurrent infections (for example, oral thrush, skin fungi, persistent herpes, atypical pneumonia)
• The development of white, hairy-looking plaques on the tongue or inner cheek (hairy leucoplakia)
• An otherwise rare form of cancer (Kaposi's sarcoma) that forms purplish-red patches that may be visible on the skin or in the mouth

AIDS progression is monitored by measuring the number of T-helper cells in the blood (CD4 count) and the number of circulating viral particles (viral load).

How are HIV and AIDS diagnosed?
Normally, production of antibodies would help to wipe out an infection, but because HIV hides away inside T-helper cells, it remains safe from antibody attack and may remain dormant for many years. The presence of anti-HIV antibodies acts as a marker that someone has been exposed to the HIV virus and forms the basis of the HIV test.

An HIV test shows whether or not you have been exposed to the human immunodeficiency virus. It can take up to three months after infection before someone taking an HIV test to become HIV-positive. So if you're worried about a recent risk of exposure, you may be advised to wait for three months from the date of possible exposure to have the test.

Sometimes you will be offered a blood test that will be 'saved', so that if a subsequent test does prove positive, the previous test can also be checked. This might help to show whether or not the possible exposure you were worried about (e.g. an assault) was the one that transmitted the HIV infection.

If the result comes back positive, you will be offered another blood test to recheck the results, as occasionally false-positive results may occur. If the checks confirm that you do have anti-HIV antibodies, you will be informed that you are HIV positive. The results of the test are almost always given to you face to face, so there is someone to talk you through the results and their full implications.

How are HIV and AIDS treated?
Antiviral drugs may be given to help reduce viral replication. Other treatments (e.g. antifungal drugs, anti-diarrhoeal drugs) may be given if other AIDS-related infections develop.

Protecting yourself from HIV

• Use condoms
• Avoid sex abroad
• Avoid obviously risky partners
• If using intravenous drugs, never share needles

In one study, only one in ten partners of HIV-positive people who used condoms became HIV positive themselves, compared with 12 in 14 partners infected where condoms were not used. The spermicide, nonoxynol-9, adds protection against both pregnancy and infection - it destroys some disease-causing organisms, including HIV, in the same way that it kills sperm.

The use of both the oral contraceptive pill and a condom - the so-called double-Dutch method - provides protection against pregnancy while also reducing the risk of a sexually transmissible infection.

Male homosexuals should select extra strong condoms for maximum protection as these are less likely to burst during use, especially if used with a water-based lubricating gel.

Receiving zidovudine following a needle-stick injuries or other exposures may help to reduce the risk of passing on infection.

If you're worried you have an STI or want to find out more, check out the symptoms, diagnosis and treatments for:

• Chlamydia
• Genital herpes (HSV)
• Genital warts
• Gonorrhoea
• Pubic lice
• Scabies
• Syphilis
• Trichomonas