Sexually transmitted infections and diseases (STIs and STDs)

Sexually transmitted infections (STIs) are infections that can be passed from partner to partner during sexual activity. STIs are caused by an organism, such as a bacteria, a virus, or a parasite, and are most likely to be transmitted through unprotected oral, vaginal, or anal sex. In some cases, STIs can be passed on through skin-to-skin contact.

What are sexually transmitted infections?

Sexually transmitted infections (STIs) are infections that are spread through sexual activity.

Examples of STIs include:

  • chlamydia
  • gonorrhea
  • hepatitis B virus
  • herpes simplex virus type 2
  • human immunodeficiency virus (HIV)
  • human papillomavirus (HPV)
  • syphilis
  • trichomoniasis

As of 2018, around 20% of people in the US had an STI, although as many as 75% of people may experience HPV at some point in their lives. HPV is the most prevalent sexually transmitted infection. Just four of these STIs — chlamydia, trichomoniasis, HPV, and genital herpes — made up 98% of cases in 2018. They also represented 93% of new cases the same year.

What’s the difference between STIs and STDs?

STIs and STDs are often referred to interchangeably. Some institutions and organizations differentiate between the two, noting that STIs are infections that can be spread to other people. Once you have an STI, it may develop into a chronic disease (STD).

One example is HIV, an STI that may become acquired immunodeficiency syndrome (AIDS) if the infection isn’t treated.

Sexually transmitted infections and male fertility

STIs like chlamydia, HIV, and HPV may reduce sperm count and quality, possibly contributing to 15% of cases of male infertility. However, more research needs to be done to determine the link between STIs and male fertility.

If an STI is transmitted to a partner, it can also cause infertility and pregnancy complications.

How to prevent STIs

Condoms are highly effective at preventing some STIs, while others, such as genital herpes, can be spread by skin-to-skin contact even when using a condom. You can get vaccines to prevent hepatitis B and HPV (most people receive these vaccines when they’re children or preteens). And a medication called pre-exposure prophylaxis (or PrEP) can, when taken regularly, lower the chances of contracting HIV.

It’s also important to undergo regular STI testing, especially if you’re not in a monogamous relationship, and to promptly treat infections to prevent further issues.

Back to Glossary