Sexually Transmitted Infections (STIs): Types, Symptoms, Testing, Treatment and Prevention

Sexually transmitted infections, commonly called STIs, are infections that can spread primarily through sexual contact.

This can include:

• vaginal sex,
• anal sex,
• oral sex,
• and some forms of intimate skin-to-skin contact.

Some infections can also spread through blood or from a pregnant person to their baby during pregnancy, childbirth or, for certain infections, breastfeeding.

STIs are extremely common worldwide.

According to the World Health Organization (WHO), more than 30 different bacteria, viruses and parasites are known to be transmitted through sexual contact.

Importantly, someone can have an STI without knowing it.

Many sexually transmitted infections cause no noticeable symptoms, particularly during their early stages.

Testing is therefore an important part of sexual healthcare.

STI or STD: What's the Difference?

You may encounter both STI and STD.

STI means sexually transmitted infection.

STD means sexually transmitted disease.

The term STI is increasingly used because someone can carry an infection without developing symptoms or disease.

For example, someone may have chlamydia without experiencing symptoms.

That person still has an infection that can potentially be transmitted.

What Are the Most Important STIs?

WHO identifies eight pathogens associated with a particularly large burden of sexually transmitted infection.

Four are currently generally curable:

• chlamydia,
• gonorrhoea,
• syphilis,
• and trichomoniasis.

Four important viral infections are:

• HIV,
• human papillomavirus (HPV),
• herpes simplex virus (HSV),
• and hepatitis B.

Let's look at each one.

1. Chlamydia

Chlamydia is a bacterial STI caused by Chlamydia trachomatis.

It can spread through:

• vaginal sex,
• anal sex,
• and oral sex.

One of the challenges with chlamydia is that many infected people have no symptoms.

When symptoms occur, they can include:

• unusual vaginal discharge,
• discharge from the penis,
• pain or burning during urination,
• pelvic or lower abdominal pain,
• pain during sex,
• or rectal symptoms depending on the site of infection.

Untreated chlamydia can cause complications.

In women, infection can spread into the reproductive organs and contribute to pelvic inflammatory disease (PID), which can increase the risk of infertility and ectopic pregnancy.

Chlamydia can be treated and cured with appropriate antibiotics.

Sexual partners may also need testing and treatment to reduce reinfection.

2. Gonorrhoea

Gonorrhoea is caused by the bacterium Neisseria gonorrhoeae.

It can infect areas including the:

• genitals,
• rectum,
• and throat.

Some people have no symptoms.

When symptoms occur, they can include:

• unusual vaginal discharge,
• discharge from the penis,
• pain or burning during urination,
• pelvic discomfort,
• rectal symptoms,
• or throat symptoms.

Untreated gonorrhoea can cause serious complications, including reproductive-health problems and infertility.

Gonorrhoea is treatable with antibiotics.

However, antimicrobial resistance is a major global concern.

WHO warns that gonorrhoea has developed resistance to multiple classes of antibiotics, making correct diagnosis and treatment increasingly important.

Do not attempt to treat suspected gonorrhoea with leftover antibiotics.

3. Syphilis

Syphilis is a bacterial STI caused by Treponema pallidum.

The infection progresses through different stages and can produce different symptoms.

Early syphilis may cause a painless sore at the site where infection entered the body.

Later symptoms can include:

• skin rash,
• swollen lymph nodes,
• fever,
• headache,
• tiredness,
• and other symptoms.

Symptoms can disappear even though the infection remains in the body.

Without appropriate treatment, syphilis can eventually damage important organs.

Syphilis during pregnancy can also cause severe complications for the developing baby.

Syphilis can be cured with appropriate antibiotic treatment.

Testing is important because symptoms can easily be missed.

4. Trichomoniasis

Trichomoniasis is caused by a parasite called Trichomonas vaginalis.

It is one of the most common curable STIs worldwide.

Many infected people have no symptoms.

Women who develop symptoms may experience:

• vaginal discharge,
• genital itching or irritation,
• discomfort during urination,
• or pain during sex.

Men are frequently asymptomatic, although some can develop:

• discharge from the penis,
• irritation,
• or pain during urination.

Trichomoniasis is treatable and curable with appropriate medication.

Partners may need treatment to prevent reinfection.

5. Genital Herpes

Genital herpes is caused by herpes simplex virus, or HSV.

There are two main types:

• HSV-1,
• HSV-2.

HSV-1 commonly causes oral herpes or cold sores but can also cause genital herpes, including through oral-genital contact.

HSV-2 is primarily sexually transmitted and is a major cause of genital herpes.

Many people with herpes have no symptoms or symptoms so mild that they do not recognize them.

When symptoms occur, they can include:

• painful blisters,
• ulcers or sores,
• itching,
• tingling,
• or burning.

The virus remains in the body and symptoms can recur.

There is currently no cure that eliminates herpes from the body, but antiviral medicines can:

• reduce symptoms,
• shorten outbreaks,
• and help reduce recurrence and transmission risk.

Herpes can sometimes be transmitted even when no visible sores are present.

6. Human Papillomavirus (HPV)

Human papillomavirus, or HPV, refers to a large group of related viruses.

HPV is extremely common.

Many infections cause no symptoms and clear naturally.

Some HPV types can cause genital warts.

Other high-risk HPV types can cause cancers, including:

• cervical cancer,
• anal cancer,
• penile cancer,
• vulval cancer,
• vaginal cancer,
• and some cancers of the throat.

Having HPV does not mean someone will develop cancer.

Persistent infection with certain high-risk types is what creates the greatest concern.

HPV Vaccination

Vaccination is one of the most important ways to prevent disease caused by high-risk HPV types.

HPV vaccines can protect against HPV types responsible for many HPV-related cancers.

Countries have different:

• vaccination schedules,
• eligibility ages,
• catch-up programmes,
• and cervical-screening recommendations.

Check the current recommendations from the health authority where you live.

People who have been vaccinated should still follow applicable cervical-screening recommendations.

7. HIV

Human immunodeficiency virus (HIV) attacks cells involved in the immune system.

HIV can be transmitted through certain body fluids, including during sexual contact.

It can also be transmitted through:

• contaminated blood,
• sharing injecting equipment,
• and from mother to child during pregnancy, childbirth or breastfeeding.

HIV cannot be diagnosed reliably from symptoms alone.

Testing is necessary.

HIV Is Not the Same as AIDS

HIV and AIDS should not be listed as two separate sexually transmitted infections.

HIV is the virus.

AIDS—acquired immunodeficiency syndrome—describes the most advanced stage of HIV infection when significant immune-system damage has occurred.

Modern antiretroviral treatment can prevent HIV from progressing to AIDS.

Can HIV Be Treated?

Yes.

There is currently no widely available cure for HIV, but modern antiretroviral therapy (ART) can control the virus extremely effectively.

With appropriate treatment, people living with HIV can live long and healthy lives.

Modern HIV treatment has transformed the outlook for people living with the infection.

Undetectable = Untransmittable (U=U)

This is one of the most important developments in modern HIV care.

When someone living with HIV takes effective antiretroviral treatment and maintains an undetectable viral load, they do not sexually transmit HIV to their partners.

This is commonly expressed as:

Undetectable = Untransmittable (U=U).

WHO recognizes the evidence supporting this principle.

People receiving HIV treatment should continue appropriate medical monitoring.

8. Hepatitis B

Hepatitis B is a viral infection that affects the liver.

The hepatitis B virus can spread through:

• sexual contact,
• infected blood,
• contaminated needles,
• and from mother to baby during childbirth.

Some people develop an acute infection and recover.

Others develop chronic hepatitis B.

Long-term infection can increase the risk of:

• liver cirrhosis,
• liver failure,
• and liver cancer.

A highly effective hepatitis B vaccine is available.

Vaccination is an important prevention strategy worldwide.

What About Scabies?

The original article listed scabies as one of the 13 STDs.

This needs clarification.

Scabies is caused by a tiny mite called Sarcoptes scabiei.

It spreads mainly through prolonged skin-to-skin contact.

Because sexual activity often involves prolonged close skin contact, scabies can certainly spread between sexual partners.

However, it is not exclusively sexually transmitted.

People can acquire scabies through other prolonged close physical contact.

Symptoms commonly include:

• intense itching,
• particularly at night,
• and a characteristic rash.

Scabies is treatable.

Close contacts may need treatment at the same time to prevent reinfestation.

What About Pubic Lice?

Pubic lice—sometimes called crabs—are small parasitic insects that typically live in coarse body hair, particularly pubic hair.

They commonly spread through close physical or sexual contact.

They are not caused by bacteria or viruses.

Pubic lice can be treated, and clothing, bedding and towels may also need appropriate cleaning.

Sexual or close contacts may need to be informed and treated.

Is Vaginal Yeast Infection an STI?

Usually, no.

This is another important correction to the original article.

Vaginal yeast infection, or vulvovaginal candidiasis, is generally caused by overgrowth of Candida yeast that can normally be present in the body.

It is not generally classified as a sexually transmitted infection.

Symptoms can include:

• vaginal itching,
• soreness,
• irritation,
• discomfort during sex,
• and abnormal discharge.

Because several STIs can cause similar symptoms, professional assessment may be appropriate when the cause is uncertain.

Is Pelvic Inflammatory Disease an STI?

PID itself is not normally classified as an STI.

Pelvic inflammatory disease is an infection/inflammation of the upper female reproductive tract that can develop as a complication of infections, including untreated chlamydia and gonorrhoea.

PID can affect structures such as the:

• uterus,
• fallopian tubes,
• and ovaries.

Possible symptoms include:

• lower abdominal or pelvic pain,
• abnormal vaginal discharge,
• pain during sex,
• bleeding between periods,
• or fever.

Some people have mild symptoms.

Untreated PID can lead to complications including:

• infertility,
• chronic pelvic pain,
• and ectopic pregnancy.

Early diagnosis and treatment are important.

Why Can STIs Be Difficult to Recognize?

One of the biggest misconceptions about STIs is that an infected person will always have obvious symptoms.

That is not true.

Many infections can be asymptomatic.

A person may therefore:

• feel completely healthy,
• have no visible sores,
• have no unusual discharge,
• and still have an infection.

This is why looking at a partner cannot determine whether that person has an STI.

When Should You Consider STI Testing?

Testing may be appropriate if:

• you have symptoms that could indicate an STI,
• a sexual partner has been diagnosed with an STI,
• you have had sex where an STI could have been transmitted,
• you have a new sexual partner,
• you have multiple partners,
• a healthcare professional recommends routine screening,
• or you are pregnant and screening is recommended.

Testing recommendations depend on:

• age,
• sexual practices,
• pregnancy,
• number of partners,
• local infection rates,
• and individual risk.

A healthcare professional or sexual-health service can advise which tests are appropriate.

How Are STIs Tested?

Different infections require different tests.

Testing may involve:

• urine,
• blood,
• vaginal swabs,
• cervical swabs,
• urethral swabs,
• rectal swabs,
• throat swabs,
• or samples from sores.

The test needed depends partly on the infection and the sites exposed during sexual activity.

For example, someone who has had oral or anal sex may sometimes require throat or rectal testing in addition to genital testing.

Can You Test Too Soon?

Yes.

STIs have different window periods—the period between acquiring an infection and when a test can reliably detect it.

Testing immediately after a possible exposure may therefore not always rule out infection.

A healthcare professional or testing service can advise:

• when to test,
• which test to use,
• and whether repeat testing is needed.

What Should You Do If You Test Positive?

A positive STI result is a medical issue—not a moral judgment.

Follow the treatment or management plan recommended by your healthcare professional.

Depending on the infection, this may involve:

• taking medication,
• avoiding sexual contact temporarily,
• informing recent sexual partners,
• partner testing,
• partner treatment,
• follow-up testing,
• or additional screening.

Treating partners can be important for preventing reinfection and interrupting transmission.

Can STIs Be Cured?

Some can.

WHO identifies four major curable STIs:

• chlamydia,
• gonorrhoea,
• syphilis,
• and trichomoniasis.

Other viral infections may not currently be curable but can often be prevented, treated or managed effectively.

For example:

• HIV can be controlled with ART,
• herpes symptoms can be managed with antivirals,
• hepatitis B can be prevented through vaccination and chronic infection can be medically managed,
• and HPV vaccination can prevent many important HPV infections and associated cancers.

How Can You Reduce Your Risk of STIs?

No single strategy is appropriate for everyone, but prevention can include:

• using condoms correctly and consistently,
• having appropriate STI testing,
• discussing sexual health with partners,
• limiting exposure to new infections,
• HPV vaccination,
• hepatitis B vaccination,
• avoiding sharing injecting equipment,
• and obtaining appropriate HIV prevention when indicated.

Condoms and STI Prevention

Correct and consistent condom use is one of the most effective ways to reduce the risk of many sexually transmitted infections, including HIV.

However, condoms do not provide complete protection against every STI.

Infections such as:

• herpes,
• syphilis,
• HPV,
• scabies,
• and pubic lice

may involve skin outside the area covered by a condom.

Condoms still provide important protection and should not be dismissed simply because protection is not absolute.

HIV PrEP

Pre-exposure prophylaxis (PrEP) is medicine used by people who do not have HIV to substantially reduce their risk of acquiring HIV.

WHO currently recommends several PrEP options, although which products are available differs between countries.

PrEP prevents HIV; it does not prevent other STIs.

People using PrEP should follow the testing and medical-monitoring programme recommended by their healthcare provider.

HIV PEP

Post-exposure prophylaxis (PEP) is emergency medication that can reduce the risk of HIV infection after a potential exposure.

PEP needs to be started quickly.

If you think you may have been significantly exposed to HIV, contact an appropriate healthcare service as soon as possible rather than waiting for symptoms.

Local availability and procedures differ between countries.

Pregnancy and STIs

Some sexually transmitted infections can affect pregnancy or be transmitted to a baby.

Potential consequences can include:

• pregnancy complications,
• premature birth,
• stillbirth,
• neonatal infection,
• and other serious health problems.

Screening during pregnancy is therefore important.

Recommendations vary internationally, so pregnant people should follow the testing recommendations provided by their healthcare professionals.

Avoid Stigma

Anyone who is sexually active can potentially acquire an STI.

Having an STI does not determine someone's:

• character,
• cleanliness,
• morality,
• or worth.

Stigma can discourage people from seeking testing and treatment.

Sexual-health conversations should therefore be factual, respectful and confidential.

The Bottom Line

Sexually transmitted infections are caused by numerous bacteria, viruses and parasites.

Important STIs include:

• chlamydia,
• gonorrhoea,
• syphilis,
• trichomoniasis,
• genital herpes,
• HPV,
• HIV,
• and hepatitis B.

Many infections cause no symptoms, which means testing can be important even when someone feels healthy.

Some STIs are curable.

Others can be effectively treated, controlled or prevented.

Modern prevention includes much more than simply telling people to “be safe.” It can involve:

• condoms,
• regular testing when appropriate,
• partner communication,
• vaccination,
• HIV PrEP,
• HIV PEP,
• and prompt treatment.

Most importantly, people concerned about a possible STI should seek appropriate testing rather than trying to diagnose themselves from symptoms alone.

Medical Disclaimer

This article provides general educational information for an international audience and is not a substitute for diagnosis, STI testing or individualized medical advice. Screening recommendations, vaccines, medicines, PrEP, PEP and sexual-health services differ between countries. Consult an appropriate healthcare professional or sexual-health service for advice based on your circumstances.

Sources and Further Reading

World Health Organization — Sexually Transmitted Infections (STIs)

https://www.who.int/news-room/fact-sheets/detail/sexually-transmitted-infections-(stis)

World Health Organization — HIV and AIDS

https://www.who.int/news-room/fact-sheets/detail/hiv-aids

World Health Organization — Herpes Simplex Virus

https://www.who.int/news-room/fact-sheets/detail/herpes-simplex-virus

World Health Organization — Gonorrhoea

https://www.who.int/news-room/fact-sheets/detail/gonorrhoea-(neisseria-gonorrhoeae-infection)

World Health Organization — Trichomoniasis

https://www.who.int/news-room/fact-sheets/detail/trichomoniasis