HIV is primarily transmitted during sexual intercourse. Blood, semen, vaginal fluid and pre-ejaculate contain the virus, which can easily be passed between partners. The risk of infection is particularly high during anal sex.
In 2012, it was estimated that 22,600 HIV positive individuals in the UK were unaware of their infection (approximately 21% of all cases). The infection can go unnoticed for a long period of time, which is why HIV testing is highly recommended if you have had unprotected sex.
The HIV virus can also be transmitted via blood transfusion (although in the UK, all donated blood is checked for HIV so there is almost no risk of infection). Breast milk of infected mothers may contain the virus and which can be transmitted to the baby during breastfeeding.