Sexually-transmitted infections

Sexually transmitted infections (STIs) are passed from one person to another through sexual contact. This includes anal, oral or vaginal sex. There are more than 30 different pathogens that cause STIs. These include bacteria like Chlamydia and viruses like HIV. To reduce the spread of STIs people can abstain from sexual activity or use a barrier-type of contraception like a condom.


A picture of a human cell infected with chlamydia bacteria. The bacteria are	much smaller than the human cell.
A human cell infected by the bacteria of the STI Chlamydia (the green substance in the centre).

Chlamydia is a sexually transmitted infection caused by a bacterium. It is a common infection, especially amongst people under 25. The NHS recommends that people who are under 25 and sexually active have a Chlamydia test every year or if they change partner.

Chlamydia causes a burning pain when urinating and often forms a thick yellow or green discharge from an infected person's penis or vagina. In women it can also cause bleeding between periods and men can develop swollen testicles.

If untreated, infection with Chlamydia can result in infertility. Like other bacterial infections, Chlamydia is treated by antibiotics.


HIV stands for human immunodeficiency virus. This infection is transmitted by body fluids, often during unprotected sex, but also through cuts and injecting drugs using unsterilized needles. Immediately after infection, people often suffer mild flu-like symptoms. These pass and for a period of time infected people might not know they are infected.

AIDS stands for acquired immune deficiency syndrome. Months or years after the infection of the HIV virus, the virus may become active and start to attack the patient’s immune system. HIV at this point has resulted in AIDS.

There is no cure for HIV /AIDS although many scientists are trying to find one. Currently, infected people are given antiviral drugs, which can significantly slow the development of AIDS.

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