Ch 5 · Sexually Transmitted Infections · IGCSE Biology 0610
Topic 5 of 5 in Reproduction — IGCSE Biology 0610 — 58 lessons.
Introduction
Sexually transmitted infections are examined through definitions, methods of transmission and methods of control, with HIV as the named example. Candidates describe how the virus spreads, how spread can be limited and, at the Extended level, how the virus damages the immune system so that infection may progress to AIDS. Use the correct terms: pathogen, transmission, lymphocyte, antibody.
Key ideas — Definition of an STI
A sexually transmitted infection (STI) is an infection that is transmitted through sexual contact. STIs are caused by pathogens of different kinds: bacteria (for example gonorrhoea, syphilis, chlamydia), viruses (HIV, hepatitis B, genital herpes) and others. The pathogen passes from one person to another in body fluids exchanged during sexual contact. Bacterial STIs can usually be treated with antibiotics; viral STIs cannot.
🧑🏫 Teacher's explanation
Think of an STI like a 'hitchhiker' that only uses a specific type of vehicle to move from one person to another. In this case, the 'vehicle' is sexual contact. Just as some hitchhikers might be easily removed from a car (like bacteria being cleared by antibiotics), others are much harder to get rid of because they hide inside the car's engine (like viruses hiding inside your cells).
📖 Definitions
🔍 Worked example — Easy
Classify the following pathogens into 'Bacterial' or 'Viral' categories: Gonorrhoea, HIV, Syphilis, Hepatitis B.
🔍 Worked example — Standard
A patient is diagnosed with an infection that cannot be treated with antibiotics. Explain why this suggests the pathogen is a virus rather than a bacterium.
🔍 Worked example — Exam-style
Explain how the HIV virus leads to the development of AIDS in terms of the immune system.
⚠️ Common errors
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🎯 Exam question
Key ideas — HIV and AIDS
Human immunodeficiency virus (HIV) is a pathogen that causes an STI. HIV infection may lead to AIDS (acquired immune deficiency syndrome). A person may carry HIV for years with few symptoms; AIDS is the later stage in which the immune system is so weakened that the person suffers repeated infections and certain cancers that a healthy immune system would prevent. There is no vaccine and no cure, but antiretroviral drugs slow the replication of the virus and can delay or prevent progression to AIDS.
🧑🏫 Teacher's explanation
Think of your immune system as a highly trained security team in a building. The lymphocytes are the specialized guards who recognize specific intruders and produce 'wanted posters' (antibodies) to catch them. HIV is like a master infiltrator that doesn't just steal things; it specifically targets and destroys the security guards themselves.
📖 Definitions
🔍 Worked example — Easy
State the specific type of cell targeted by HIV and the consequence of this destruction on the immune system.
🔍 Worked example — Standard
A patient is diagnosed with HIV. Explain why they may appear healthy for several years despite being infected with a pathogen.
🔍 Worked example — Exam-style
Compare the use of antiretroviral drugs with a vaccine in the management of HIV.
⚠️ Common errors
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🎯 Exam question
(a) State what is meant by the term 'pathogen'. [1] (b) Describe how HIV affects the immune system and explain why this makes the patient susceptible to secondary infections. [3] [4 marks]
Key ideas — Transmission of HIV
HIV is carried in blood, semen, vaginal fluid and breast milk. It is transmitted by:
🧑🏫 Teacher's explanation
Think of your immune system as a highly trained security team in a building. To defend the building, the team needs specialized agents called lymphocytes. These agents create specific 'wanted posters' called antibodies to catch intruders. HIV is like a saboteur that doesn't just break in; it specifically targets and destroys the security agents themselves.
📖 Definitions
🔍 Worked example — Easy
Identify two body fluids (other than blood) that can transmit HIV.
🔍 Worked example — Standard
Explain why a person with HIV is more susceptible to common respiratory infections.
🔍 Worked example — Exam-style
A patient's lymphocyte count was 1500/μ extL last year. This year, it is 400/μ extL. Calculate the percentage decrease in lymphocyte count.
⚠️ Common errors
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🎯 Exam question
(a) State two ways HIV can be transmitted. [2] (b) Explain how HIV leads to a reduced ability of the body to fight off other infections. [3] [5 marks]
Key ideas — Controlling the spread of STIs
Because STIs are transmitted in body fluids, control methods aim to prevent the exchange of those fluids or to identify and treat infected people.
🧑🏫 Teacher's explanation
Think of an STI like a highly contagious 'glitch' in a computer network. If the glitch travels through specific data cables (body fluids), you can stop it by unplugging the cables (barrier methods) or by installing a firewall (vaccination/education) or by scanning the system for the glitch and patching it (testing and treatment).
📖 Definitions
🔍 Worked example — Easy
Identify two different methods of preventing the transmission of HIV through blood-to-blood contact.
🔍 Worked example — Standard
Explain why antiretroviral drugs are used in the management of HIV infection.
🔍 Worked example — Exam-style
A patient is diagnosed with an STI caused by a bacterium. Compare the treatment for this bacterial STI with the treatment for HIV.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) State one method used to prevent the transmission of this infection during sexual contact. [1] (b) Explain why antibiotics are ineffective against this specific infection. [2] [3 marks]
Key ideas — How HIV affects the immune system
🧑🏫 Teacher's explanation
Think of your immune system as a highly organized military command center. To fight an invading army (pathogens), you need more than just soldiers; you need generals to coordinate the attack. In your body, these generals are a specific type of white blood cell called helper T-lymphocytes. They don't fight the battle directly, but they send out chemical signals to tell B-lymphocytes to start mass-producing antibodies.
📖 Definitions
🔍 Worked example — Easy
Identify the specific cell type targeted by HIV and state the primary consequence of this destruction on the immune response.
🔍 Worked example — Standard
A patient's lymphocyte count is monitored over time. At month 6, the count is 1200 cells/μ extL. At month 24, the count is 300 cells/μ extL. Calculate the percentage decrease in lymphocyte count.
🔍 Worked example — Exam-style
Explain why a person with advanced HIV/AIDS is highly susceptible to opportunistic infections like pneumonia.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the type of cell targeted by the HIV virus. [1] (b) Explain how the destruction of these cells leads to the development of AIDS. [3] [4 marks]
Worked example
(a) Sharing needles contaminated with infected blood [B1]; mother to child across the placenta, during birth or in breast milk (or transfusion of unscreened blood) [B1].
🔍 Worked example — Easy
Identify the biological difference between HIV and AIDS.
🔍 Worked example — Standard
Calculate the percentage decrease in lymphocyte count if a patient's lymphocyte count drops from 1500 ext cells/μ extL to 300 ext cells/μ extL.
🔍 Worked example — Exam-style
Explain why a person with AIDS is susceptible to 'opportunistic infections' like fungal pneumonia.
⚠️ Common errors
Common mistakes
Practice
Questions