HIV can be transmitted in only four body fluids: blood, semen, vaginal fluids/secretions and breast milk. An infected person can only infect someone else if the virus in these fluids enters the blood of the other person. This can happen in five key ways:

• Unprotected penetrative sex: anal and vaginal unprotected (without a barrier contraception such as a condom) sex carries a high risk of transmission, where only one person is positive. The presence of other sexually transmitted infections may increase the possibility of transmission.

• Re-using and sharing needles, syringes and works: can lead to the exchange of small quantities of blood. Injecting drug-users who share works can transmit the virus in this way. Re-using needles in a medical setting, which still occurs in some developing countries, can also lead to transmission. Needle stick injuries that can occur to health care workers and others working in places where used needles are likely can transmit HIV. Click here to open an HPA poster detailing first aid following an occupational exposure.

• Infected blood and blood products: people having blood transfusions in the UK and most other developed countries are not at risk because all blood is screened. In some developing countries where there is no blood screening, there is a risk.

• Unprotected Oral Sex: carries a lower risk of transmission than anal or vaginal sex without a condom, though exactly how risky continues to be the subject of research and controversy. Bleeding gums, ulcers or infections in the mouth/throat can increase the risk. It is also suggested that the quantity of infectious fluid entering the mouth may be significant, whilst the key factor is likely to be the viral load in semen or vaginal secretions. If an infected woman is menstruating the risk may increase slightly as a result of the presence of blood. Oral sex without a condom or barrier is more risky in terms of other sexually transmitted infections such as Hepatitis B and gonorrhoea.

• Mother to child: There are three points at which an HIV-positive mother might infect her baby. During pregnancy - via the placenta. During delivery - HIV may infect the baby in the mother's cervical secretions, or blood during childbirth, or through breast feeding after the birth. In developed countries, about 14% of children born to mothers who are HIV positive and untreated for their HIV infection will themselves be infected. This figure rises to about 33% in less developed countries. A review of 8,533 births in Europe and North America established that the risk of transmission was reduced from 7.3% to 2% by the use of caesarean section before the rupture of membranes and onset of labour. The rate of transmission was reduced by 87% in women who received antiretroviral therapy plus elective caesarean delivery compared to women who delivered in other ways and had no therapy.

HIV cannot be transmitted through ordinary social contact such as sharing cups, from toilet seats or from shaking hands.

• Condoms used properly offer reliable protection during vaginal and anal sex. If using a lubricant, oil based ones such as Vaseline and massage oil should never be used with latex condoms, as it compromises the barrier properties of latex; use water-based lubricants, such as KY or Foreplay. There are different types of condom available for vaginal and anal intercourse. The spermicidal lubricant Nonoxynol 9 may possibly destroy HIV, but at the same time it can cause irritation and damage to the vaginal wall, increasing the risk of HIV transmission. Visit our condoms pages.

• The femidom, or female condom, is inserted into the vagina, and if used properly offers reliable protection. It is a lubricated sheath made of polyurethane, which will not be destroyed by oils. The femidom is sometimes used for protection during anal sex.

• Condoms and oral shields provide protection during oral sex.

• There are numerous sexual activities where there is absolutely no risk of HIV transmission; these include kissing, licking, massage, mutual masturbation.

PEP is a course, generally for at least a month, of anti-retroviral medication given after exposure to HIV in order to try and prevent the person becoming HIV positive. This is available on request in the UK to healthcare workers who have had a needlestick injury. Availability for other exposures is varied, and is generally linked to knowledge of the HIV status of the person from whom infection could have come.

PEP must be given within 72 hours of exposure, and the sooner after exposure the greater the effectiveness of the intervention.

PEP should be available from your HIV clinic or out of hours from your accident and emergency department. PEP should be available following sexual exposure if a) your partner is known to be HIV positive b) Following sexual assault and in some circumstances where the partner comes from a high risk group for HIV. Policy varies from hospital to hospital.

Remember PEP is not a substitute for avoiding exposure in the first place, and HIV drugs can be toxic and have severe side effects.

The comments below are not restricted to the context of HIV transmission, but also cover safer drug use in general. This is particularly important in the current situation with the uncertainty surrounding the transmission of Hepatitis C. Precautions taken against other viruses such as HIV are not considered to be sufficient protection against Hepatitis C.

• Even if taken in pill or capsule form drugs may still increase risk of HIV transmission due to the effects of the drug on the sense of awareness, increasing the potential to have unsafe sex.

• Drug injection equipment (works), which includes needles, syringes, spoons, filters and water, should not be shared.

• If possible, use new works for every injection.

• Needle exchange schemes provide free clean needles and syringes and in some cases other equipment such as sterile water. If a needle exchange scheme cannot be located, it is possible to obtain needles and syringes from accident and emergency departments, although this is not encouraged.

• As an absolute last resort if new equipment is not available, it is possible to clean dirty works with cold or warm water (do not use hot water as this hardens the blood, making it more difficult to remove) and/or detergent and/or bleach. Whilst this should provide some protection against HIV it is not considered sufficient to protect against Hepatitis C.

• Always dispose of used works safely.

• Always filter street drugs - draw the liquid up into the works through an unused cigarette filter or similar to filter out the impurities.

• Use the smallest possible needle - 1mm is best.

• Never inject in the artery - draw back into the syringe a little and if the blood that flows out is bright red rather than dark red, you have hit an artery and need to withdraw immediately and apply pressure. Seek medical help.

• Change sites around the body to give veins a chance to rest.

• If you are injecting into the arms or hands, never wear rings as this can cut off the blood supply even more and increase the likelihood of gangrene.