Sexually Transmitted Infections

What makes an infection an STI

A sexually transmitted infection is an infection that can be transmitted through sexual contact. That phrase describes a possible route of transmission, not the only one — some pathogens that cause STIs can also be transmitted by entirely different routes, which matters for how their spread is actually controlled.

HIV and AIDS

HIV — human immunodeficiency virus — is a pathogen that can cause an STI. HIV infection may lead to AIDS, acquired immunodeficiency syndrome, but the two terms are not synonyms: HIV is the pathogen and the infection itself, while AIDS is a separate, later condition that HIV infection may progress to, not something that happens the moment infection occurs.

How HIV is transmitted

HIV is transmitted when infected body fluids enter another person’s body — it is not spread by ordinary casual contact such as touching, sharing everyday objects, or simply being near someone socially. The routes that do transmit it are specific: sexual contact involving the exchange of infected body fluids; sharing contaminated needles or syringes, which transfers infected blood directly between people; infected blood entering another person’s bloodstream, including through blood that hasn’t been properly screened; and transmission from an infected mother to her baby during pregnancy, birth, or breastfeeding.

Controlling the spread of STIs

Effective control always connects back to the specific route a pathogen actually uses to spread, rather than applying one generic measure to every situation. Correct and consistent condom use reduces the risk of transmission for many STIs by reducing contact with infectious fluids or tissues during sexual contact. Testing identifies infection, which allows treatment where it’s available and reduces the chance of onward transmission through safer behaviour. Avoiding shared needles or syringes, using sterile medical equipment, and screening donated blood all interrupt the blood-borne route specifically. And education about how transmission actually happens helps people avoid genuine risks without generating unnecessary stigma around forms of everyday contact that carry no risk at all.

A useful way to hold this together for an exam answer is to match the measure to the route rather than listing them separately: infected blood is addressed by sterile equipment and screened blood supplies; exposure to infected sexual fluids is addressed by barrier methods, reduced exposure, and testing; mother-to-child transmission is addressed by medical prevention and treatment strategies specific to pregnancy and birth. Naming a control measure without connecting it to the route it actually interrupts is usually where a mark is lost here, not a lack of biological knowledge.

Where this fits

This closes the human-reproduction pathway that runs from the reproductive system through fertilisation and pregnancy— sexual contact is one of several routes this chapter has already described substances and cells passing between two people, and it’s worth seeing STIs as one more consequence of that same route rather than an unrelated topic bolted onto the end of the chapter.