STI basics: most are common, treatable and preventable

An STI is a sexually transmitted infection: a bacterium, virus or parasite that can pass between people during sex. STD, sexually transmitted disease, is the older term and is still common in the US. The World Health Organization (opens in a new tab) and the NHS (opens in a new tab) say STI, and Planned Parenthood (opens in a new tab) uses both. The shift towards “infection” reflects something useful to know: many infections never turn into a disease or cause a single symptom. You can carry one and feel completely well.

This page uses STI, except where a source or a common search says STD. They mean the same infections.

STIs are among the most common infections there are. The World Health Organization (2025) (opens in a new tab) estimates that more than 1 million curable STIs are acquired every day worldwide among people aged 15 to 49, and that in 2020 there were 374 million new infections with one of four curable STIs: chlamydia, gonorrhoea, syphilis and trichomoniasis. More than 30 different bacteria, viruses and parasites can pass through sexual contact, but eight account for the greatest number of infections, and four of those eight are curable.

Those numbers are not there to alarm anyone. They are the reason testing is routine rather than remarkable, and why a positive result is a health matter to sort out rather than a verdict on anyone. Planned Parenthood (opens in a new tab) puts it simply: STDs are really common, lots of people who have them have no symptoms, getting tested is no big deal, and most are easy to treat. Most STIs are common, treatable and preventable, and routine testing is normal in the lifestyle.

Preventable is the other half. Condoms and other barriers lower the risk of most STIs, two have safe and highly effective vaccines, and there is a daily or injectable medicine that greatly reduces the chance of getting HIV. None of these makes risk zero. Together they make it low and manageable, which is what playing safe means.

One caution from WHO is worth knowing. Drug resistance in gonorrhoea has risen quickly in recent years and has narrowed the treatment options. That is not a reason to worry about catching it; it is a reason to finish any treatment exactly as prescribed, to avoid sex until you and your partners have finished treatment, as the NHS (opens in a new tab) advises, and to follow your clinician’s advice about retesting.

If you test positive, the NHS advises telling current and recent partners so they can be tested and treated too, and notes that clinics can often contact them for you without naming you. In the lifestyle, where people keep in touch after events, a calm, early message is a kindness.

The other four of WHO’s eight main infections are viruses: HPV, herpes, HIV and hepatitis B. According to the WHO fact sheet (opens in a new tab), antiviral medicines cannot yet cure herpes or HIV but can change the course of both, and antivirals for hepatitis B help fight the virus and slow liver damage. Two of the four have vaccines.

HPV deserves a special word because it is so often misunderstood. The World Health Organization (2024) (opens in a new tab) says almost all sexually active people will be infected at some point, usually without symptoms, and that the immune system usually clears it within a year or two. A small number of persistent high-risk infections can lead to cancer over years, which is what the vaccine and cervical screening are for. Our guide to vaccines for sexual health covers who can get the HPV and hepatitis B vaccines.

For HIV, the CDC’s PrEP page (2026) (opens in a new tab) describes PrEP as pills or shots that greatly reduce the chance of getting HIV from sex, and is clear that it does not prevent other STIs. Whether PrEP suits you is a question for a clinician, and our guide to PrEP for HIV prevention walks through it.

The single most useful fact on this page: the World Health Organization (2025) (opens in a new tab) says the majority of curable STIs cause no symptoms, and the NHS (opens in a new tab) says many STIs have none, so you can have one without knowing and pass it to a partner. Feeling fine, looking fine and being certain you are fine are three different things.

When symptoms do appear, the NHS lists the common ones: unusual discharge from the vagina, penis or anus; pain when peeing; lumps, blisters, sores or warts around the genitals or anus; a rash; itching; and unusual vaginal bleeding. It also notes that how soon they appear depends on the infection, and that some infections can take weeks to show up on a test. Those signs are a reason to get checked, not to self-diagnose, since several different infections, and some conditions that are not STIs at all, can look alike.

Because symptoms are unreliable, testing is the tool. The CDC’s screening recommendations (2024) (opens in a new tab) are risk-based: at least one HIV test for everyone aged 13 to 64, and chlamydia and gonorrhoea tests for sexually active women under 25 and for older women with a new partner, more than one partner, or a partner with other partners. There is no official schedule written for the lifestyle; some couples choose to test every few months and after new partners as a community habit. Our testing guide covers what to ask for, window periods and testing as a couple.

The World Health Organization (opens in a new tab) says STIs pass through sexual contact, including vaginal, anal and oral sex. It helps to think of two routes. Planned Parenthood (opens in a new tab) explains that some STIs are carried in body fluids such as semen, vaginal fluids and blood, while others pass just from skin-to-skin touching with an infected area.

That is the whole reason condoms are described as highly effective rather than perfect. WHO calls condoms, used correctly and consistently, one of the most effective ways to protect against STIs, including HIV, and notes they protect less against infections that cause sores outside the covered area, such as syphilis or genital herpes. Its HPV fact sheet (opens in a new tab) makes the same point about HPV: condoms help but do not cover all the genital skin. WHO still recommends using them for vaginal, oral and anal sex where possible.

Oral sex is often treated as risk-free, and it is not. Planned Parenthood (opens in a new tab) explains that oral sex is much lower risk for HIV than vaginal or anal sex, but that herpes, syphilis, hepatitis B, gonorrhoea and HPV can all pass during oral sex. Our answer on STDs and oral sex goes further.

One common condition is often confused with an STI. Bacterial vaginosis is, according to the NHS (opens in a new tab), not classed as an STI, but sex can trigger it or pass it on, and the CDC (opens in a new tab) names new or multiple partners among the things that raise the risk. It is common and treatable; see BV and new partners.

1. Layer 01

   Talk first.

   Before you play, say what protection you use, when you last tested, and what you are and are not comfortable with. In the lifestyle this is ordinary etiquette, not an interrogation. A person who will not have the conversation has answered the question.

   

2. Layer 02

   Test routinely.

   Most STIs cause no symptoms, so testing is the only way to know. CDC's screening advice is risk-based rather than one schedule for everyone; some couples choose to test every few months and after new partners as a community habit. Share results with partners as a dated snapshot, not a guarantee.

3. Layer 03

   Use barriers.

   Condoms for vaginal and anal sex, and condoms or dental dams for oral sex. Gloves for hands. Planned Parenthood adds a toy rule: wash toys before they touch another person, or put a fresh condom on for each person.

4. Layer 04

   Vaccinate.

   Safe, highly effective vaccines exist for hepatitis B and HPV. Hepatitis A and mpox vaccines are there for people at higher risk. One conversation with a clinician covers all four.

5. Layer 05

   Know the medicines.

   PrEP lowers the chance of HIV. Doxy PEP, an antibiotic taken after sex, is one CDC recommends clinicians discuss with a narrow group, and it is still being studied for others. Both are clinician decisions, and neither replaces the layers above.

One more habit from Planned Parenthood’s safer-sex guidance (opens in a new tab) is worth borrowing for events. Keep your drinking moderate, because it is harder to use condoms well and to stick to your own limits when you are drunk or high.

On doxy PEP specifically: the CDC’s doxy PEP page (2026) (opens in a new tab) says taking the antibiotic doxycycline within 72 hours after sex can help some people lower their chances of chlamydia, syphilis and, in some studies, gonorrhoea; that it does not prevent HIV or herpes; and that CDC recommends clinicians discuss the pros and cons with gay and bisexual men and transgender women who have had one of those infections in the past year. Researchers are still learning whether it works for other groups. Our doxy PEP guide explains what that means for everyone else.

The NHS (opens in a new tab) suggests visiting a sexual health clinic if you think you might have an STI, if a partner has symptoms, if you are worried after sex without a condom or dental dam, if you are pregnant and have symptoms, or if you are having sex with new partners. The last one covers most people reading this, which is exactly the point: a routine check is ordinary care, not a sign that something is wrong.

A visit is usually simpler than people expect. The NHS describes a few questions about your sex life, perhaps a look, and a urine sample, a blood test or swabs. It notes that clinic staff are there to help and have heard everything, that you do not have to give your real name at an NHS sexual health clinic, and that what you share stays within the clinic unless you ask otherwise. In the US, a doctor, a local health department or a clinic such as Planned Parenthood can test you; ask about privacy and costs when you book.

Be specific about what you do. If you have oral or anal sex, say so, so the clinician can decide whether to take a throat or rectal swab as well as a urine or blood sample. And if a test is positive, the next steps are treatment and telling partners, both of which your clinic can help with.

A last word on trust. A verified profile (on JoinTheSwing once it opens, or anywhere else) confirms who someone is. It does not tell you anything about their sexual health or their consent. Those come from direct conversation, dated results shared between partners, and protection you choose for yourself.