STD basics

What is an STD?


A sexually transmitted disease (STD) is an infectious illness that is transmitted through unprotected sexual activity...including vaginal, anal and oral sex, or skin-to-skin contact with an infected area. STDs can also be spread via skin-to-skin contact or through intravenous (IV) drug use, and can be passed to an unborn child during pregnancy or through breastfeeding. 

STDs are caused by some 30 viruses, bacteria and parasites that survive and spread through sexual contact. STDs have been around for hundreds of years and most people will experience an infection during their lifetime...the Centers for Disease Control and Prevention (CDC) estimate 19 million new cases every year, which presents a major public health challenge. Indeed, untreated STDs are estimated to cause at least 24,000 women to become infertile every year.

STDs vs. STIs

Diseases that are passed from person to person through sexual activity are sexually transmitted diseases (STDs). "Disease" implies a clear medical problem, usually with obvious signs or symptoms. But some common STDs are asymptomatic; in these cases, the sexually transmitted virus or bacteria can be described as causing a sexually transmitted infection (STI), which may or may not evolve into a "disease." Because "STD" is preferred by most people, we generally use that term for both sexually transmitted infections and diseases.

Last reviewed by Lisa Oldson, MD, January 2011.

Lisa Oldson, MD

STD expert

"The first thing I tell a patient about STDs is that if you're worried about one STD, you should probably worry about all STDs. In other words, if you had unprotected sex and you're worried about a possible HIV exposure, it's important to understand that hepatitis can be spread in the same fashion...ditto for chlamydia, gonorrhea, herpes and syphilis."

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