About Sexually Transmitted Infections (STIs)

At a glance

  • Sexually transmitted infections (STIs), also known as sexually transmitted diseases (STDs), are very common.
  • Millions of new infections occur every year in the United States.
  • STIs are preventable. If you have sex, know how to protect yourself and your sex partner(s) from STIs.
4 different types of couples abstracted with pathogens of STIs and prevention-related concepts


A sexually transmitted infection (STI) is a virus, bacteria, fungus, or parasite people can get through sexual contact. A sexually transmitted disease (STD) develops because of an STI and the term implies that the infection has led to some symptom of disease. People sometimes use the terms in one another’s place. The primary goal of public health and healthcare is to prevent and treat infections before they develop into disease. As a result, many – including CDC – are using the term STI more often. However, STD is still used when referring to data or information from sources that use the term.

Signs and symptoms

Many STIs have no symptoms or may only cause mild symptoms, so people can have an infection but not know it. Therefore, it is possible to have an infection and not know it. That is why getting an STI test is important if you are having sex. If you receive a positive STI diagnosis, know that all are treatable with medicine and some are curable entirely.

How it spreads

STIs pass from one person to another through vaginal, oral, and anal sex. They also can spread through intimate physical contact like heavy petting, though this is not very common.

Related conditions

Some related conditions which are often transmitted sexually include

Other conditions are sometimes but not always spread by sexual transmission, such as

Other diseases, including Zika, Ebola, and mpox, can be spread sexually but are more often spread through ways other than sex.