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. They can also be passed through anal sex, oral sex, or skin-to-skin contact. More than 9 million women in the United States are diagnosed with an STI each year. Women often have more serious health problems from STIs than men, including infertility. The organisms (bacteria, viruses or parasites) that cause sexually transmitted diseases may pass from person to person in blood, semen, or vaginal and other bodily fluids. Sometimes these infections can be transmitted nonsexual, such as from mother to infant during pregnancy or childbirth, or through blood transfusions or shared needles.