• Sexual transmission. This is the most common way the virus is passed on. You can become infected if you have sex with an infected person (vaginal, anal or oral sex). Semen, vaginal secretions and blood from an infected person contains HIV. The virus can enter the body through the lining of the vagina, vulva, penis, rectum, or mouth during sex. You can also become infected from using 'sex toys' previously used by an infected person which have not been washed properly.
• Needle sharing. HIV and other viruses such as hepatitis B and C virus can be transmitted by drug users who share needles, syringes and other injecting equipment which is contaminated with infected blood.
• Infected blood. In the past quite a number of cases occurred from infected blood transfusions and other blood products. This is now rare in the UK as all blood products are checked for HIV before being used. It is still a significant problem in developing countries.
• Accidental needlestick injuries. The risk is low but some cases have developed in health care workers who have been injured accidentally by contaminated needles.
• From mother to child. About 1 in 3 untreated pregnant women with HIV pass the infection on to their babies during pregnancy or childbirth. However, treatment with anti-HIV drugs during pregnancy greatly reduces the risk of passing on the virus. Having a caesarean section to deliver the baby reduces the risk even further. HIV can also be passed to babies through breast milk during breastfeeding. If formula milk is available, mothers with HIV are encouraged not to breast feed.
• Rare causes. Some cases have been reported where HIV has been transmitted through organ or tissue transplants, and by contaminated medical or dental equipment which has not been sterilised properly.

Note: To become infected with HIV, some infected blood, semen or vaginal secretions has to get into your body. You cannot catch HIV from ordinary contact with someone with HIV such as hugging, shaking hands, touching, etc, or from sharing food, towels, utensils, swimming pools, telephones, etc. It is also thought that kissing does not transmit the virus.

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