HIV is spread by:
- Having sex without a condom. HIV infection can happen through anal, vaginal or oral sex if you don’t use a condom. Unprotected (condom-less) oral sex is not as risky as vaginal and anal, but still can spread HIV, especially when there are cuts, bleeding gums or canker sores in the mouth.
- Sharing needles, syringes or drug works. Sharing any of the equipment to inject drugs can spread HIV.
- Pregnancy, childbirth or breastfeeding. Without treatment, an HIV positive woman will transmit HIV to her child during pregnancy or childbirth about 25% of the time. Babies also become positive through breastfeeding.
Knowing how HIV transmission happens is the first step to preventing it.
Source: http://www.aidshealth.org/learn-about-it.html/2/