An STI, or sexually transmitted infection, is an infection that can pass through sexual contact. Viruses, bacteria, fungi, and parasites can all cause STIs. An infection may cause no symptoms, so a person can have and pass on an STI without realizing it.

CDC explains that an STD develops because an STI has led to symptoms or disease. People often use the terms interchangeably, and many health agencies now use STI because it includes infections without symptoms. The practical point is that testing can identify an infection, and a clinician can explain treatment and partner-care steps for the infection found.

A common misconception

STI and STD are related terms rather than two unrelated groups. CDC explains that an STD describes an STI that has led to symptoms or disease, while an infection can exist without noticeable symptoms.

How STIs are found

Testing depends on the infection and the body site involved. A clinician may use urine, a swab, blood, or another sample. Tell the clinician about oral, vaginal, or anal contact so they can discuss which sites need testing. FDA-approved self-collection options exist for some infections, but availability and eligibility vary.

What to do if you are concerned

Talk with a healthcare professional after a possible exposure, if a partner has an STI, or when a clinician recommends screening. If a test is positive, follow the prescribed treatment and ask what follow-up and partner care are needed.

An online symptom checker cannot tell you whether you have an STI. Symptoms overlap, and no symptoms does not rule an infection out.

What a definition cannot tell you

The word STI does not identify a particular infection, body site, or treatment. A clinician uses your exposure, symptoms, and test results to decide which evaluation and follow-up apply.

Sources and update note

Checked September 29, 2026. This guide uses CDC About STIs and CDC Getting Tested for STIs. Use current local guidance and a healthcare professional for decisions about your situation.

Community option

After getting reliable medical information, some people want peer support for disclosure or dating. The optional community link below is separate from diagnosis and medical care.