Sexually transmitted infections (STIs) are infections you can get by having sex with someone who has an infection. These infections are usually passed from person to person through vaginal intercourse, anal sex, oral sex, or skin-to-skin contact. STIs can be caused by viruses or bacteria. STIs caused by viruses include Hepatitis B, Herpes, HIV, and the human papilloma virus (HPV). STIs caused by bacteria including chlamydia, gonorrhoea, and syphilis.