Oral sex is the stimulation of the genitals using the mouth and tongue. It is one of the ways that sexually transmitted infections (STIs) are most frequently passed on. You can catch an STI if you have just one sexual partner. However, the more partners you have, the greater the risk of catching an infection. STIs commonly caught through oral sex are: gonorrhoea genital herpes syphilis Infections less frequently passed on through oral sex include: chlamydia HIV hepatitis A , hepatitis B and hepatitis C genital warts pubic lice