Sexually Transmitted Infections STI, STD

Authored by , Reviewed by Dr John Cox on | Certified by The Information Standard

This leaflet gives some brief information about sexually transmitted infections (STIs) and what you are advised to do if you suspect that you have an STI. An STI is sometimes referred to as a sexually transmitted disease (STD) or venereal disease (VD). This leaflet also gives some information about genitourinary medicine (GUM) clinics, sometimes called sexual health clinics. For example, how you can find a GUM clinic and what you can expect when you go to one.

A sexually transmitted infection (STI) is an infection that can be passed from person to person when having sex. You can get an STI by having vaginal sex, anal sex, or oral sex. There are several different types of STI. 

The ten most common STIs in the UK are anogenital warts, chlamydia, genital herpes, gonorrhoea, human immunodeficiency virus (HIV), hepatitis B, hepatitis C, pubic lice, syphilis and trichomonas.

Anogenital warts are small lumps that develop on the genitals and/or around the back passage (anus). They are sometimes just called genital warts. They are caused by a germ (virus) called the human papillomavirus (HPV). However, most people infected with HPV do not develop visible warts. You can carry the virus without knowing it and can pass it on to other people who may then develop warts. Treatment options include applying chemicals to the warts or freezing the warts to destroy them. Read more about anogenital warts.

Chlamydia is caused by a germ (bacterium) called Chlamydia trachomatis. It is the most common STI in the UK. Symptoms include a vaginal discharge in women and a discharge from the penis in men. You can be infected with chlamydia for months, even years, without realising it, as it often causes no symptoms. However, even if you have no symptoms, you can still pass on the infection and complications may develop if it is left untreated (such as pelvic infection and infertility in women). A short course of an antibiotic medicine clears chlamydia in most cases. Learn more about chlamydia.

Genital herpes is caused by the herpes simplex virus. Once you catch this virus it stays with you for life but lies dormant without causing symptoms for most of the time. In fact, many people who are infected with this virus never have symptoms. If symptoms occur, they can range from a mild soreness to many painful blisters on the vulva or penis and surrounding area. A first episode of symptoms can last 2-3 weeks but may be shorter. Recurrent episodes of symptoms then develop in some cases from time to time but are usually less severe than the first episode. (It is similar to having cold sores but on the genitals from time to time.) Antiviral medication can ease symptoms when they develop. Discover more about genital herpes.

Gonorrhoea is caused by a bacterium called Neisseria gonorrhoeae. Symptoms include a vaginal discharge in women and a discharge from the penis in men. Again, some people infected with gonorrhoea do not develop symptoms. However, even if you have no symptoms, you can still pass on the infection and complications may develop if it is left untreated (such as pelvic infection and infertility in women). A short course of an antibiotic clears gonorrhoea in most cases. Find out more about gonorrhoea.

Human immunodeficiency virus (HIV) is most commonly passed on by sexual contact. HIV attacks cells of the immune system. Over time (usually several years) the immune system weakens so that you cannot defend your body against various bacteria, viruses and other germs. This is when acquired immunodeficiency syndrome (AIDS) develops. Many infections and conditions can develop if you have AIDS. Treatment with antiretroviral medicines can reduce the viral load of HIV and allow your immune system to work effectively. However, treatment does not clear the virus from the body. Therefore, if you are infected with HIV, you will need monitoring for the rest of your life and treatment is long-term. Read more about HIV and AIDS.

Hepatitis B is a virus that primarily attacks the liver. The virus is mainly passed on by sexual contact, sharing contaminated needles to inject street drugs, or from an infected mother to her baby. The hepatitis B virus can cause a short-term (acute) infection, which may or may not cause symptoms. Following an acute infection, some people develop a persistent infection called chronic hepatitis B. Many people with chronic hepatitis B remain well but can still pass on the virus to others (as they are carriers). Some develop serious liver problems. If needed, antiviral medication may prevent or reduce the severity of liver inflammation and liver damage. See the separate leaflet called Hepatitis B.

Hepatitis C is a virus that primarily attacks the liver. Most cases occur in people who share needles contaminated with traces of infected blood, in order to inject street drugs. There is a small risk that an infected person can pass on the virus whilst having sex. Some people clear the infection naturally. Some people with persistent (chronic) infection remain free of symptoms but some have symptoms. After many years of infection, some people develop a severe scarring of the liver (cirrhosis) and some develop liver cancer. Treatment is difficult but it can clear the infection in up to half of cases. See the separate leaflet called Hepatitis C.

Pubic lice (often called crabs) are tiny insects about 1-2 mm long (smaller than a match head). They lay eggs which hatch into lice after seven days. Pubic lice attach strongly to hairs and do not wash or brush off with normal cleaning. Pubic lice are passed on by close bodily contact, especially when having sex. The main symptom is itch, usually in the pubic hair area. However, you may not have any symptoms but may still pass on the lice to others. Treatment with a lotion or cream usually clears the lice. See the separate leaflet called Pubic and Body Lice.

Syphilis is caused by a bacterium called Treponema pallidum. If it is not treated, it can spread in the bloodstream from the genital region to cause various symptoms and problems in different parts of the body over many years. A short course of antibiotics usually clears syphilis infection. Learn more about syphilis.

Trichomonas is a protozoan. This is a small germ a bit like a bacterium. It can cause an infection that is not normally serious but symptoms can be unpleasant. Symptoms include a vaginal discharge in women and a discharge from the penis in men. Some people infected with trichomonas do not have symptoms but can still pass on the infection. A course of antibiotics usually clears trichomonas infection. Discover more about the protozoan called trichomonas.

There are some other STIs that are uncommon in the UK - for example, donovanosis and chancroid.

Sexually Transmitted Infections

You may not be able to think logically at a time when you most need to. But you will not be alone. Anyone who is sexually active can acquire an STI and there are trained professionals whose job it is to help you.

— Dr Laurence Knott, What to do if you think you have an STI

Editor's note

June 2018 - Dr Hayley Willacy has just read the latest report from Public Health England - see Further reading below. This reports 422,147 new diagnoses of STIs by sexual health services in England in 2017. The most commonly diagnosed STI was chlamydia, which accounted for nearly half of all new STI diagnoses. First-episode genital warts accounted for about 1 in 7 of all new STI diagnoses, while gonorrhoea accounted for about 1 in 9 new diagnoses. Both syphilis and gonorrhoea diagnoses increased, but the number of genital warts cases fell in young women. This is largely because of the high coverage of the HPV vaccination programme for girls of school age.

Scabies is a common skin rash that is caused by a tiny insect (mite). It is usually very itchy. You need close skin-to-skin contact with an infected person to catch scabies. Most cases are probably caught from holding hands with an infected person. However, sleeping in the same bed and sexual contact are other common ways of passing on the mite which is why some people regard scabies as an STI. See the separate leaflet called Scabies.

Molluscum contagiosum is a common skin rash that is caused by a germ (virus). It is passed on by skin-to-skin contact. The rash consists of small lumps which are pearly white or slightly pink. Each lump (molluscum) looks like a small wart and is round, firm and about 1-5 mm across. Sometimes the virus is passed on during the close contact of having sex. So, some people regard molluscum contagiosum as an STI. If it is passed on whilst having sex then the first mollusca to appear tend to be on the skin around the penis or vagina.

However, many cases of molluscum contagiosum are not caused by a sexual contact but by simply touching other affected people. It is a common condition that is seen both in adults and in children. See the separate leaflet called Molluscum Contagiosum.

These terms describe the site of a problem rather than a particular cause of the problem.



Urethritis means inflammation of the urethra. The urethra is the tube that passes out urine from the bladder. If you have urethritis you may develop a burning sensation when you pass urine and men may have a discharge from the end of the penis. Urethritis is usually caused by an STI such as chlamydia or gonorrhoea (see above) but can also be caused by injury. See the separate leaflet called Urethritis and Urethral Discharge in Men and read more about non-gonococcal urethritis.


Balanitis means inflammation of the tip of the penis. Balanitis is sometimes caused by an STI. However, it is most commonly caused by non-sexually transmitted infections and skin conditions. Balanitis is common in young children, due to non-sexually transmitted infections. See the separate leaflet called Balanitis.

Pelvic inflammatory disease (PID)

PID is an infection of the womb (uterus). Germs (bacteria) that cause the infection usually travel into the womb from the vagina or neck of the womb (cervix). An STI is a common cause of PID. Most cases are caused by chlamydia or gonorrhoea. However, some cases are not due to an STI. Symptoms of PID include pain in the lower tummy (abdomen), high temperature (fever), abnormal vaginal bleeding and a vaginal discharge. Possible complications include infertility, persistent pain and an increased risk of an ectopic pregnancy if you become pregnant. See the separate leaflet called Pelvic Inflammatory Disease.


Vulvitis means inflammation of the vulva. (The vulva is the skin just outside the vagina.) There are various causes of vulvitis, including some STIs. See the separate leaflet called Vulvitis.

The genitals (vagina and vulva in women, penis in men) can be affected by various other conditions. Some people are confused as to what is and what is not caused by an STI. For example, the following are not usually due to an STI:

Symptoms of each STI can vary, from local symptoms affecting the genitals, to symptoms that affect various other parts of the body. The following is not a full list of all possible symptoms. However, these are the common symptoms to look out for:

  • A vaginal discharge.
  • Abnormal vaginal bleeding.
  • A discharge from the penis.
  • A sore, ulcer, rash, or lump that appears on the penis or around the vagina, vulva or back passage (anus).
  • Pain when you have sex.
  • Pain when you pass urine (although the common reason for this is a urine infection and not an STI).
  • Swelling of the glands in your groin.

But remember, in many cases of STI, no symptoms may develop. However, you can still pass on the infection to others even if you have no symptoms. Therefore, if you think that you may have an STI, it is best to get it checked out. 


of people would visit their GP if they they had an STI. 37% would visit a sexual health clinic.

Source: Patient Sexual Health Survey

If you have a symptom that you think is due to an STI, or if you have no symptoms but are worried you have caught an STI then you should see a health professional.

How do you find out if you have an STI?

Dr Rosemary Leonard MBE

You can:

  • See your own GP. Your GP can give advice and may examine you. However, if your GP suspects that you may have an STI, in the UK he or she is likely to refer you to a GUM clinic. Some GPs may do tests and manage the situation without a referral to a GUM clinic; or
  • Go to a GUM clinic directly. In the UK you do not need a referral from your GP to go to a GUM clinic. You can ring the local hospital or health authority and ask where the nearest clinic is. Local and national information is also available on the internet - for example, from the Family Planning Association's 'Find a clinic' service.

Until you are checked out, and treated if necessary, you should not have sex. This is to prevent you passing on any infection.

GUM clinics are special clinics that help people who have, or may have, an STI or certain other problems with their genitals or urinary system.

Depending on the initial assessment and examination, the doctor may advise on some tests and ask for your consent to do the tests. Learn more about tests for STIs.

If you are diagnosed with an STI then the clinic will encourage you to tell any current or recent sexual partners that you have an infection. You are not obliged to do this or to give out any information about other people. However, it is best that any recent sexual partners should know that they might also be infected. They should be offered testing and treatment if necessary, to prevent the infection being spread any further. This telling of sexual partners is sometimes called contact tracing. If you prefer, clinics can contact people anonymously if you do not wish to tell them yourself. You should be aware that recklessly exposing a sexual partner to the risk of infection is against the law.

The treatment that you will be offered depends on which STI is found. For example, a short course of antibiotics can usually clear away chlamydia, gonorrhoea, syphilis and trichomonas. A cream or lotion can clear pubic lice and scabies. Topical treatments can usually clear most anogenital warts. Treatments for genital herpes, hepatitis B, hepatitis C and HIV are more involved and complex. You will be given advice about what treatment options you have and given time to ask questions.

If you are prescribed antibiotics then it is important to finish the full course of tablets, or else the infection may not be fully cleared. If you develop side-effects then seek advice from the GUM clinic or from your GP as to what to do. Do not simply stop taking the medication. For some infections you will be asked to return after a course of treatment to check that the infection has gone.

Do not have sex again until the time advised by the clinic. Depending on the infection, this may be for a certain length of time after treatment has finished or it may be until you are given the all clear from a repeat test. The aim is to prevent you from passing on the infection to others.

Advice from a sexual health adviser

Most clinics will have a sexual health adviser. You are likely to be seen by the adviser in addition to being seen by a doctor or nurse. A sexual health adviser is specially trained and can:

  • Tell you more about STIs and how to avoid catching them in future. For example, the risk of infection increases with the number of changes of sexual partner. Wearing a condom during sex helps to prevent against STIs.
  • Give you tips on how to cope with any current symptoms.
  • Offer you free condoms which can help to prevent STIs.
  • Give advice about what to tell your sexual partner or previous partners.
  • Help you with contacting previous sexual partners who should be tested or treated. For example, you may be given a card with a number on it, plus a printed message advising the person who is given the card to visit the clinic and to bring the card along. This helps to match them with your notes.

Other services

In addition to diagnosing and treating STIs, if needed, a GUM clinic can also usually:

  • Do a pregnancy test.
  • Arrange counselling if you are pregnant and are not sure what to do.
  • Arrange counselling about the decision to have an HIV test.
  • Provide advice about contraception.
  • Provide emergency contraception.
  • Do a cervical smear test on women if one is due.
  • Diagnose and treat some other conditions of the genitals that are not sexually transmitted. For example, a urine infection, thrush and some skin conditions affecting the genital area.

Further reading and references

What young people really want from sex education
The best way to treat a herpes outbreak
Safeguarding your sexual health at university

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