STI — an infection passed on by sexual contact
Both halves of the definition are marked: infection (a pathogen is involved — not a general health condition) and transmitted through sexual contact (the route that makes it sexually transmitted, rather than airborne, waterborne or food-borne). HIV (human immunodeficiency virus) is the only STI pathogen named in Cambridge 0610, and it is a virus — so HIV is the worked example behind almost every question in this sub-topic.
HIV transmission — three routes, all fluid exchange
Unprotected sexual intercourse — HIV is present in genital fluids and passes to a partner when no condom is used. Blood-to-blood contact — most often by sharing needles or syringes, so infected blood enters another person's bloodstream. Mother to child — during pregnancy, at birth, or through breastfeeding. Every route is an exchange of body fluids between two people — which is why touching, sharing food and airborne spread are not routes.
STI control — the syllabus triad
0610 limits STI control to three measures. Restricting the number of sexual partners — fewer opportunities for the infection to be passed on. Using a condom — a barrier stopping the exchange of genital fluids during intercourse. Screening and treatment — testing finds infected people, including those with no symptoms, so treatment protects them and cuts onward transmission. Education about all three underpins public-health campaigns.
Drawn from real examiner reports.
HIV is a virus, not a bacterium
Examiners saw a wide range of answers for the type of pathogen responsible for HIV infection, with many candidates stating it is bacterial. The V in HIV is virus — so no antibiotic will treat it. If the question asks for the type of pathogen, the answer is the single word "virus"; writing "HIV" simply repeats the name given in the question and does not answer it.
HIV pathogen type given as bacterium — s22 Paper 31 Q2b i, June 2022 sitting (digest Section 10 records the same misconception under a different topic heading).
AIDS is not the pathogen — HIV is
Many candidates wrote AIDS as the name of the pathogen. AIDS (acquired immune deficiency syndrome) is a condition — the advanced stage reached once HIV has destroyed enough lymphocytes to disable the immune system. A syndrome is not an organism, so "AIDS" can never answer "name the pathogen that causes this STI". HIV is the cause; AIDS is the possible consequence.
AIDS given as the pathogen — s22 Paper 31 Q2b i, June 2022 sitting.
HIV is not airborne or casual contact
The report notes many candidates suggesting routes with no relationship to a sexually transmitted infection — airborne spread, sharing food, touching, coughs and sneezes. These are respiratory or casual-contact routes borrowed from unrelated diseases. HIV needs an exchange of body fluids, so shaking hands, sharing a cup or breathing the same air transmits nothing.
Non-STI transmission routes suggested (airborne, sharing food, touching, coughs/sneezes) — s22 Paper 31 Q2b ii, June 2022 sitting.
"Sexual transmission" is too vague
Among candidates who did correctly link HIV to sexual contact, many wrote only "sexual transmission" and stopped there. The mark scheme wants the extra detail: transmission occurs through unprotected intercourse — that is, without a condom. The word "unprotected" is doing real work, because protected intercourse is exactly the case the barrier control measure removes.
"Sexual transmission" stated without the unprotected-sex / no-condom detail — s22 Paper 31 Q2b ii, June 2022 sitting.
"Through blood" without the mixing
Naming blood as a route earns nothing on its own: infected blood must mix with another person's blood, for example through a shared needle carrying it into the bloodstream. Being near or touching infected blood does not transmit HIV. Examiners also saw "sharing injections" — name the shared needle or syringe itself, since that object is the actual route.
"Blood" given without stating that blood must mix with another person's blood; "sharing injections" written instead of the needle/syringe — s22 Paper 31 Q2b ii, June 2022 sitting.
Answers stop short of naming AIDS
Asked for the consequence of a falling lymphocyte count in an HIV-positive patient, candidates gave very detailed answers, yet the development of AIDS was rarely mentioned. "Lymphocytes fall so the person is less healthy" earns partial credit at best. Finish the chain: fewer lymphocytes, weaker immune system, this stage is AIDS, other infections take hold.
Development of AIDS omitted from lymphocyte-decline answers — s22 Paper 42 Q4b iii, June 2022 sitting (as cited in the shared digest).
Name the route the measure blocks
Use the scaffold [measure] [route it interrupts] [why spread falls]: "a condom prevents the exchange of genital fluids, blocking the sexual route for HIV". A bare "it stops the disease spreading" fits any measure, so it scores nothing.
Three measures, three mechanisms
Restricting partners is behavioural (fewer opportunities); a condom is a barrier (no fluid exchange); screening is detection (symptomless cases found and treated). Give the mechanism belonging to the measure named — one generic sentence will not score three marks.
Finish the chain through to AIDS
Extended lymphocyte questions need every link: HIV destroys lymphocytes fewer antibodies weaker immune system this stage is AIDS infections the body would normally control take hold. Never stop at "becomes ill".
Sexually transmitted infection (STI): an infection that is transmitted through sexual contact.
Human immunodeficiency virus (HIV): the STI pathogen named in this syllabus -- HIV is a virus (not a bacterium).
Acquired immune deficiency syndrome (AIDS): (Extended) the advanced stage of HIV infection, reached once the number of lymphocytes has fallen so much that the body's immune system can no longer effectively fight off other infections and diseases.
State three ways HIV can be transmitted between people.
(a) Define the term "sexually transmitted infection (STI)."
(b) State the type of pathogen that causes HIV/AIDS.
