Infectious Diseases – Cambridge IGCSE/A‑Level Biology (9700)
1. What is an infectious disease? (LO 10.1.1)
- Diseases caused by transmissible pathogens – organisms that can be passed from one host to another.
- Pathogen types listed in the syllabus: bacteria, protoctists, viruses, fungi, helminths.
- Non‑infectious diseases (e.g., genetic disorders) are not caused by transmissible agents.
2. Pathogens and the diseases they cause (LO 10.1.2)
| Disease | Pathogen (name) | Pathogen type (syllabus terminology) | Typical transmission mode (LO 10.1.3) |
|---|
| Cholera | Vibrio cholerae | Bacterium | Water‑borne (contaminated drinking water or food) |
| Malaria | Plasmodium falciparum,
Plasmodium malariae,
Plasmodium ovale,
Plasmodium vivax | Protoctist (called “protoctist” in the syllabus; an apicomplexan parasite) | Vector‑borne (bite of an infected female Anopheles mosquito) |
| Tuberculosis (TB) | Mycobacterium tuberculosis,
Mycobacterium bovis | Bacterium | Air‑borne (inhalation of aerosolised droplets that remain suspended in the air) |
| HIV/AIDS | Human immunodeficiency virus (HIV) | Virus (retrovirus) | Blood‑borne and sexual contact (exchange of infected bodily fluids) |
3. Transmission mechanisms (LO 10.1.3)
- Water‑borne: Pathogens such as V. cholerae survive in contaminated water; poor sanitation promotes spread.
- Vector‑borne: Plasmodium species develop inside Anopheles mosquitoes; control of mosquito populations reduces incidence.
- Air‑borne: Mycobacteria are expelled in tiny, aerosolised droplets that can stay airborne for long periods; ventilation and mask use limit spread.
- Blood/sexual transmission: HIV is present in blood, semen, vaginal fluid and breast milk; safe‑sex practices and sterile needles prevent infection.
4. Prevention and control – biological, social and economic factors (LO 10.1.4)
Biological interventions
- Vaccination – e.g., BCG vaccine for TB; future malaria vaccines under development.
- Chemoprophylaxis – antimalarial tablets for travellers to endemic areas.
- Effective treatment – appropriate use of antibiotics for bacterial infections; antiretroviral therapy for HIV.
Social measures
- Improving water supply and sanitation to stop cholera outbreaks.
- Health education on safe sex, needle‑exchange programmes, and the importance of completing antibiotic courses.
- Community‑based vector control (e.g., eliminating standing water, insecticide‑treated nets).
Economic considerations
- Investing in vector control yields long‑term savings by reducing malaria‑related healthcare costs.
- Funding public‑health surveillance systems enables early detection of outbreaks, limiting economic impact.
- Cost‑effectiveness of vaccination programmes versus treatment of disease complications.
5. Antibiotics – action, limitations and resistance (Topic 10.2)
How antibiotics work (focus on penicillin)
- Penicillin interferes with the synthesis of the bacterial cell‑wall peptidoglycan, causing lysis of susceptible bacteria.
Why antibiotics do not affect viruses
- Viruses lack a cell wall and most metabolic processes; they rely on host cells for replication, so antibiotics have no target.
Antibiotic resistance (LO 10.2.2)
- Enzymatic degradation – e.g., β‑lactamases break down penicillin.
- Altered target sites – e.g., mutations in penicillin‑binding proteins reduce drug binding.
- Reduced drug uptake – e.g., changes in porin channels limit entry of the antibiotic.
- Efflux pumps – e.g., active transport proteins expel the drug from the bacterial cell.
Mitigation strategies
- Prescribe antibiotics only when needed and complete the full course.
- Use narrow‑spectrum agents where possible.
- Implement infection‑control policies (hand hygiene, sterilisation of equipment).
- Surveillance of resistance patterns and public‑education campaigns.
Optional extension (for teachers who wish to enrich the topic)
- Brief overview of other antibiotic classes (e.g., tetracyclines, macrolides) and their targets (protein synthesis, DNA replication, metabolic pathways).
- Discussion of emerging resistance trends such as multidrug‑resistant (MDR) and extensively drug‑resistant (XDR) bacteria.
6. Key terminology
- Protoctist: A eukaryotic microorganism; the syllabus uses this term for the malaria‑causing Plasmodium species.
- Retrovirus: A virus that stores its genetic information as RNA and uses reverse transcriptase to integrate into the host genome (e.g., HIV).
- Vector: An organism that transmits a pathogen from one host to another without becoming diseased itself.
- Antibiotic resistance: The ability of bacteria to survive and multiply despite the presence of an antibiotic that would normally inhibit or kill them.