Sexually transmitted infections

In a nutshell

STIs are sexually transmitted infections that are spread by sexual activity. HIV is an STI caused by a virus. Chlamydia and gonorrhoea are both STIs caused by a bacterium.

HIV

The human immunodeficiency virus (HIV) is transmitted through exchanging bodily fluids. This can happen during unprotected sex, whilst injecting drugs using shared needles, and through cuts. For a few weeks, a person infected with HIV may experience flu-like symptoms. As a result, they may not know they have HIV until they experience more severe symptoms several years later.

HIV attacks the immune cells in the body which damages the immune system. This means the infected individual can not fight off disease as well as a healthy individual. At this late stage of infection, the virus has caused acquired immune deficiency syndrome (AIDS).

There is no cure for HIV, but it can be controlled using antiretroviral drugs. These slow the development of AIDS by preventing the virus from replicating in the body.

Virus lifecycle

Viruses can infect humans and they will replicate by the lytic cycle (A) or the lysogenic cycle (B).

Biology; Health, disease and the development of medicines; KS4 Year 10; Sexually transmitted infections

Lytic cycle (A)

1.	The virus attaches to the host cell and injects its genetic material (DNA or RNA) into the host cell.
2.	The viral DNA or RNA is now in the host cell.
3A.	The viral DNA or RNA is copied by the host cell. This means new viral proteins are made.
4A.	The viral DNA or RNA is packaged to make new viruses.
5A.	The new viruses are released from the host cell. This will usually destroy the host cell. These viruses can then go on to infect other cells.

Lysogenic cycle (B)

1.	The virus attaches to the host cell and injects its genetic material (DNA or RNA) into the host cell.
2.	The viral DNA or RNA is now in the host cell.
3B.	The viral DNA or RNA is inserted into the chromosomes of the host cell. At this point it is called a provirus.
4B.	The provirus will replicate with the rest of the host DNA (blue)
5B.	The provirus can stay dormant for a long time and just continue replicating with the host cells.
6B.	The viral DNA or RNA can become separated from the host DNA. The provirus is now active and it can make new viruses through the lytic cycle (following through step 2.)

Chlamydia

Chlamydia is an STI caused by a bacterium and is most commonly present in individuals under the age of $25$ . It may cause a burning sensation when urinating and a thick yellow or green discharge from an infected person's penis or vagina. For women, chlamydia can also cause bleeding between periods and for men, it can cause swollen testicles.

Chlamydia can be treated using antibiotics but if left untreated, it can result in infertility. An infected mother can pass chlamydia to her baby during birth.

Gonorrhoea

Gonorrhoea is caused by a bacterium and is most common in people aged $15-24$ . Similarly to chlamydia, gonorrhoea also causes a burning pain when urinating and a thick yellow or green discharge from an infected person's penis or vagina.

The infection can be treated with antibiotics, but if left untreated, it can result in infertility. It is recommended that a barrier-type of contraception, like a condom, is used during sexual intercourse to prevent infection.