STI — an infection passed on by sexual contact
An STI is an infection transmitted between people through sexual contact. The word "sexual" is the mark-scheme-critical part: it is what separates an STI from an infection spread by airborne droplets, contaminated food or an insect vector. A pathogen — virus, bacterium or fungus — causes an STI only if sexual contact is one of its routes between people. Cambridge 0654 Core names one example: HIV.
HIV is the virus; AIDS is what may follow
HIV (human immunodeficiency virus) is the pathogen — a virus — that causes an STI. It infects and gradually destroys the white blood cells of the immune system. Once enough have been destroyed, the body can no longer defend itself effectively against other infections, and this weakened condition is AIDS (acquired immune deficiency syndrome). HIV infection may lead to AIDS — a possible later consequence, never an instant or guaranteed one.
Three transmission routes, each with a mechanism
(1) Unprotected sexual intercourse — HIV is present in body fluids such as semen and vaginal fluid, which are exchanged during sex. (2) Contact with infected blood — sharing unsterilised needles or syringes, or transfusion with unscreened blood or blood products. (3) Mother to child — across the placenta in pregnancy, through blood contact at birth, or in breast milk. Everyday contact transmits nothing.
Drawn from real examiner reports.
"Safe sex" is not a control measure
"Practise safe sex", "be careful" and "avoid infection" name no actual method, so they score nothing. Give specific measures instead: barrier contraception (a condom) during intercourse; sterile, single-use needles, never shared; screening donated blood before transfusion; reducing the number of sexual partners; antiretroviral treatment during pregnancy.
Flagged w22 P42 Q1a ii
"Passed person to person" is too vague
"An infection you can catch from someone" would equally describe influenza, cholera or malaria, so it cannot earn the definition mark. The answer must name the specific route — sexual contact. Naming the exact route or mechanism, rather than writing something scientifically true but non-specific, is the discipline this whole sub-topic tests.
Digest cross-cutting finding, all sittings: scientifically true but off-question answers
The contraceptive pill does not stop STIs
A hormonal contraceptive such as the pill prevents pregnancy by stopping ovulation. It places no physical barrier between partners, so it does nothing to stop the exchange of body fluids and cannot control STI spread. Only a barrier method — a condom — does both jobs. "Use contraception" as an undifferentiated answer will not be credited.
"HIV always causes AIDS" overstates it
The syllabus wording is that HIV infection may lead to AIDS. Writing that an HIV-positive person therefore has AIDS, or that AIDS appears as soon as someone is infected, contradicts the specification. A person can carry and transmit HIV for years before the immune damage is severe enough for AIDS to develop.
Everyday contact does not spread HIV
HIV is not transmitted by shaking hands, hugging, sharing cups or cutlery, or being in the same room as an infected person. Answers that list these as risks, or that offer isolating infected people as a control measure, are wrong. Transmission requires sexual contact, infected blood, or the mother-to-child route.
HIV is the virus, AIDS is the syndrome
HIV is the pathogen that infects the body and passes between people. AIDS is the condition that can develop later, once HIV has destroyed enough white blood cells that the immune system can no longer fight off other infections. Writing "infected with AIDS" or "AIDS is a virus" reverses cause and effect and loses the mark. The chain runs HIV -> immune damage -> AIDS.
Digest B15 key fact, s22–w23: "HIV is a pathogen causing an STI; may lead to AIDS"
Pair every measure with the route it blocks
For "explain how the spread of HIV is controlled", answer in pairs: name the route, name the measure, then say how it works. Condom -> sexual contact -> prevents the exchange of body fluids. Sterile needle -> infected blood -> none reaches the next bloodstream.
Match the command word
"State what an STI is" = the one-line definition only. "Describe the methods of transmission" = each route named with its mechanism. "Explain how the spread is controlled" = the measure plus the reason it works. Marks follow the verb, not the volume of writing.
Give distinct routes, not three rewordings
If asked for three ways HIV is transmitted, the three must be genuinely different routes: sexual contact, infected blood, mother to child. "Sex", "unprotected sex" and "sleeping with an infected person" are one route written three ways, and two of the three marks are thrown away.
Decide first whether the answer is HIV or AIDS
Transmission and control questions are always about HIV, the virus that passes between people. Only a question about the later collapse of immune defence is about AIDS. Choosing the wrong term turns an otherwise correct answer into one that does not address the question.
Cambridge 0654 spec reference: Section B15 "Reproduction", sub-topic B15.5 (Core only -- no Extended/Supplement content is listed for this sub-topic). This leaf covers the definition of a sexually transmitted infection (STI), HIV (human immunodeficiency virus) as the named pathogen example, the possible progression from HIV infection to AIDS (acquired immune deficiency syndrome), the methods of HIV transmission, and how the spread of STIs is controlled.
Out of scope for B15.5 (do not use): the detailed biology of how HIV attacks or replicates inside white blood cells, T-helper-cell counts, antiretroviral drug mechanisms, and any named STI other than HIV -- the current syllabus names only HIV for this sub-topic.
| Term | Mark-scheme-precise definition |
|---|---|
| Sexually transmitted infection (STI) | An infection that is transmitted (passed) between people through sexual contact |
| HIV (human immunodeficiency virus) | A pathogen (virus) that causes an STI; it infects and damages the white blood cells of the immune system |
| AIDS (acquired immune deficiency syndrome) | The condition/syndrome that may develop after HIV infection has damaged the immune system enough that the body can no longer effectively fight off other infections |
HIV transmission methods (Core):
| Route | Mechanism |
|---|---|
| Unprotected sexual intercourse | HIV present in body fluids (e.g. semen, vaginal fluid) is exchanged with an infected person |
| Contact with infected blood | Sharing unsterilised needles/syringes; receiving unscreened blood or blood products by transfusion |
| Mother to child | During pregnancy (across the placenta), during birth (contact with blood), or through breastfeeding (in breast milk) |
Controlling the spread of HIV/STIs -- named measures, each linked to a route:
| Transmission route blocked | Control measure |
|---|---|
| Sexual contact | Barrier contraception (e.g. using a condom); reducing the number of sexual partners / being faithful to one tested, uninfected partner |
| Contact with infected blood | Using sterile, single-use needles (not sharing needles); screening/testing donated blood and blood products before transfusion |
| Mother to child | Antiretroviral drug treatment during pregnancy |
A vague instruction such as "practise safe sex" or "be careful" is not a named control measure and is not credited -- always name the specific action.
What is HIV, and what kind of pathogen is it?
(a) State what is meant by a sexually transmitted infection (STI). (1 mark)
(b) Name the pathogen described in the Cambridge 0654 syllabus as causing an STI, and state the type of pathogen it is (e.g. virus, bacterium). (2 marks)
