What is an STI?

An STI, or sexually transmitted infection, is basically any kind of bacterial or viral infection that can be spread through sexual contact.

Sexually transmitted infections need to be accurately diagnosed at an early stage to treat symptoms you may be experiencing and to prevent any long term complications such as infertility. It can cause a sexually transmitted disease, also referred to as a STD or Venereal disease.

Bacterial vaginosis

Bacterial vaginosis (BV) is a common yet poorly understood condition in which the balance of bacteria inside the vagina becomes disrupted. This imbalance often triggers a change to the usual vaginal discharge, which results in a fishy smelling, greyish discharge from the vagina. However, half of women with bacterial vaginosis have no symptoms. The vagina contains a mix of so-called “good” bacteria, which can help protect against infection, and a smaller amount of “bad” bacteria, which can cause infection. In cases of BV, the bad bacteria begin to outnumber the good bacteria, leading to inflammation inside the vagina, which can result in the fishy discharge.

What leads to this imbalance is still unclear. It is not classed as a typical sexually transmitted infection (STI), but it can develop after having sex with a new partner. There is no evidence that a woman with BV can pass on any type of infection to her male sexual partner. But the same may not be true for women who have sex with other women. BV is a concern if it develops in pregnant women as it increases the risk of pregnancy-related complications, such as premature birth or miscarriage. However, the increase in risk is small. Bacterial vaginosis can also increase your risk of getting some STIs. It can be diagnosed from a urine sample.

Chlamydia

Chlamydia is one of the most common STIs in the UK and is easily passed on during sex. Most people don’t experience any symptoms so are unaware they are infected. Find out more on our chlamydia page.

In women, chlamydia can cause pain or a burning sensation when urinating, a vaginal discharge, pain in the lower abdomen during or after sex, and bleeding during or after sex, or between periods. It can also cause heavy periods. In men, chlamydia can cause pain or a burning sensation when urinating, a white, cloudy or watery discharge from the tip of the penis, and pain or tenderness in the testicles. It’s also possible to have a chlamydia infection in your rectum (bottom), throat or eyes.

Diagnosing chlamydia is easily done with a urine test or by taking a swab of the affected area. The infection is easily treated with antibiotics, but can lead to serious long-term health problems if left untreated, including infertility.

Genital herpes

Genital herpes is a common infection caused by the herpes simplex virus (HSV), which is the same virus that causes cold sores.

Some people develop symptoms of HSV a few days after coming into contact with the virus. Small, painful blisters or sores usually develop, which may cause itching or tingling or make it painful to urinate.

After you’ve been infected, the virus remains dormant (inactive) for most of the time. However, certain triggers can re-activate the virus, causing the blisters to develop again, although they’re usually smaller and less painful.
It’s easier to test for HSV if you have symptoms. Although there’s no cure for genital herpes, the symptoms can usually be controlled using antiviral medicines.

Hepatitis B

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. Hepatitis B is diagnosed by blood testing.

Hepatitis C

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.  Hepatitis C is diagnosed by blood testing.

HIV

HIV is a potentially life threatening STD that requires management for the rest of your life. You can find out more about the condition on our specific HIV information page.

Gonorrhoea

Gonorrhoea is a bacterial STI easily passed on during sex.
About 50% of women and 10% of men don’t experience any symptoms and are unaware they’re infected.

In women, gonorrhoea can cause pain or a burning sensation when urinating, a vaginal discharge (often watery, yellow or green), pain in the lower abdomen during or after sex, and bleeding during or after sex or between periods, sometimes causing heavy periods.

In men, gonorrhoea can cause pain or a burning sensation when urinating, a white, yellow or green discharge from the tip of the penis, and pain or tenderness in the testicles.

It’s also possible to have a gonorrhoea infection in your rectum, throat or eyes.

Gonorrhoea can be easily diagnosed using a urine test, or by taking a swab of the affected area. The infection is easily treated with antibiotics, but can lead to serious long-term complications if left untreated, including infertility.

Mycoplasma

M. genitalium is a sexually transmitted bacterium that can cause symptoms in both men and women. Other than chlamydia and gonorrhoea, it is a main cause of urethritis (inflammation of the urethra) in men. In women, it can be associated with other infections and cervicitis (inflammation of the cervix).  Mycoplasma is diagnosed from a urine sample.

Syphilis

Syphilis is a bacterial infection that in the early stages causes a painless but highly infectious sore on your genitals or around the mouth. The sore can last for up to six weeks before disappearing. Find out more on our syphilis information page.

Secondary symptoms such as a rash, flu-like illness or patchy hair loss may then develop. These may disappear within a few weeks, after which you will have a symptom-free phase.

The late or tertiary stage of syphilis usually occurs after many years and can cause serious conditions, such as heart problems, paralysis and blindness.
The symptoms of syphilis can be difficult to recognise. A simple blood test can usually be used to diagnose syphilis at any stage. It can be treated with antibiotics, usually penicillin injections. When syphilis is treated properly, the later stages can be prevented.

Trichomonas vaginalis

Trichomonas vaginalis (TV) is an STI caused by a tiny parasite. It can be easily passed on through sex and most people are unaware they are infected.
In women, TV can cause a frothy yellow or watery vaginal discharge which has an unpleasant smell, soreness or itching around the vagina, and pain when passing urine.
In men, TV rarely causes symptoms. You may experience pain or burning after passing urine, a whitish discharge, or an inflamed foreskin.
TV can be diagnosed by a urine or swab test. Once diagnosed, TV can usually be treated with antibiotics.

Ureaplasma

Ureaplasma urealyticum is a bacterium that affects about 70% of sexually active men and women. Although it is not considered a typical STI, ureaplasma can be transmitted through sexual contact. Ureaplasma often carries no symptoms whatsoever, and most people may never experience any problems at all. However, colonies of the bacteria can multiply leading to symptoms and if left untreated can lead to complications. Ureaplasma can be diagnosed from a urine sample.