Most STIs are spread by sexual contact, by vaginal, anal or oral routes, although there does not need to be penetration or ejaculation. Infected semen or vaginal fluid getting into the eye can cause infection with chlamydia, for example. Microbes can also be transferred on sex toys that have not been washed or covered with a new condom each time they are used.1,3,4,5 
Some STIs spread via non-sexual vectors, such as blood or blood products, or by sharing used needles for injection. Seven of the principal STIs (not usually trichomoniasis) can also transfer to children from the mother either in the womb or during childbirth. STIs such as syphilis or herpes can also increase the risk of acquiring HIV by three times or more.1 
Condoms can be one of the most effective methods for protecting against STIs, including HIV, if used correctly and consistently. Dental dams offer some protection in oral sex.1,6