What causes sexually transmitted diseases (STDs) or sexually transmitted infections (STIs)?

There are three major causes of STDs/STIs:

• Bacteria, including chlamydia, gonorrhea, and syphilis
• Viruses, including HIV/AIDS, herpes simplex virus, human papillomavirus, hepatitis B virus, cytomegalovirus (CMV), and Zika
• Parasites, such as trichomonas vaginalis, or insects such as crab lice or scabies mites¹

Any STI can be spread through sexual activity including sexual intercourse, and some STIs also are spread through oral sex and other sexual activity. Ejaculation does not have to occur for an STI to pass from person to person.

In addition, sharing contaminated needles, such as those used to inject drugs, or using contaminated body piercing or tattooing equipment also can transmit some infections, such as HIV, hepatitis B, and hepatitis C. A few infections can be sexually transmitted but are also spread through nonsexual, close contact. Some of these infections, like CMV, are not considered STIs even though they can be transmitted through sexual contact.

Regardless of how a person is exposed, once a person is infected by an STI, he or she can spread the infection to other people through oral, vaginal, or anal sex, even if he or she has no symptoms.