The document discusses sexually transmitted infections (STIs), which are passed primarily through sexual contact. Some STIs cause no symptoms but can still be transmitted to partners. Untreated STIs can lead to serious health problems. Those most at risk are those who have unprotected sex, multiple partners, or partners who use intravenous drugs. STIs are caused by microbes that thrive in warm, moist areas of the body. They are transmitted through sexual acts but some can also spread through blood or from mother to child. Common STIs in the US include chlamydia, gonorrhea, syphilis, herpes, AIDS, hepatitis, genital warts, and trichomoniasis.