A sexually transmissible infection (STI) is an infection passed from one person to another through sexual contact. Most STIs, including HIV, are passed on by exchanging infected bodily fluids like semen, precum, vaginal fluids, menstrual or other blood—through unprotected oral, vaginal or anal sex. HIV and other STIs can enter the bloodstream through mucous membranes or tiny cuts, sores or abrasions on the mouth, penis, vagina, anus or hands. Some STIs are passed on during skin-to-skin genital contact.
Common STIs include chlamydia, gonorrhoea, crabs, HIV, hepatitis B, genital warts, herpes, molluscum lesions, syphilis and trichomoniasis.
STIs vary greatly in their severity, symptoms, treatments and potential for complications. They can range from having no symptoms or mild irritations in the genital area through to severe pain, fever or flu-like symptoms. Complications from some STIs include infertility, ectopic pregnancy, chronic pain, or liver damage.
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