state the name and type of pathogen that causes each of the following diseases: cholera – caused by the bacterium Vibrio cholerae, malaria – caused by the protoctists Plasmodium falciparum, Plasmodium malariae, Plasmodium ovale and Plasmodium vivax,

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)

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.