Infectious Diseases – Cambridge IGCSE/A‑Level (9700)
Learning Outcome
State that infectious diseases are caused by pathogens and are transmissible.
What the syllabus expects
The following points must be covered in the exam answer:
- Infectious diseases are caused by pathogens and can be transferred from one host to another.
- The four required syllabus diseases – cholera, malaria, tuberculosis (TB) and HIV/AIDS – and their exact causative agents.
- Primary transmission route for each of the four diseases.
- Key biological, social and economic factors that influence control of each disease.
- How penicillin kills bacteria, why it does not affect viruses, and the mechanism of antibiotic resistance.
Key Definitions
- Pathogen: Any microorganism that can cause disease (virus, bacterium, fungus, protozoan or helminth).
- Transmissible: Able to be passed from one individual (or species) to another.
- Host: Organism that provides a suitable environment for a pathogen to live, grow and reproduce.
- Susceptibility: Likelihood of a host becoming infected when exposed; affected by immunity, age, nutrition, genetics, etc.
Four Syllabus Diseases – Required Content
| Disease | Causative Agent (exact syllabus spelling) | Pathogen Type | Key Biological Feature (exam‑relevant) |
|---|
| Cholera | Vibrio cholerae | Bacterium | Produces cholera toxin (activates Gs → ↑cAMP → massive watery diarrhoea). |
| Malaria | Plasmodium falciparum (representative species of Plasmodium spp.) | Protozoan | Complex life cycle; human blood‑stage (merozoites) causes cyclic fever, gametocytes taken up by Anopheles mosquito. |
| Tuberculosis (TB) | Mycobacterium tuberculosis | Bacterium | Can persist in a latent state for years; re‑activation when immunity falls. |
| HIV/AIDS | Human immunodeficiency virus (HIV) | Virus | Integrates reverse‑transcribed DNA into host genome → chronic infection; CD4⁺ T‑cell depletion. |
General Transmission Routes
- Direct contact – skin or mucosal contact (e.g., herpes simplex).
- Indirect contact – via fomites such as clothing, bedding or surfaces.
- Droplet – large respiratory droplets expelled when coughing or sneezing.
- Aerosol – fine particles that remain suspended (e.g., measles).
- Vector‑borne – carried by arthropods (mosquitoes, ticks).
- Food & water – ingestion of contaminated substances.
- Sexual – transmission through semen, vaginal secretions or breast milk.
- Vertical – mother‑to‑child during pregnancy, birth or breastfeeding.
Primary Transmission Route for Each Syllabus Disease
| Disease | Primary Transmission Route |
|---|
| Cholera | Fecal‑oral – ingestion of water or food contaminated with V. cholerae shed in stool. |
| Malaria | Vector‑borne – bite of an infected female Anopheles mosquito injecting sporozoites. |
| Tuberculosis (TB) | Airborne droplets – inhalation of droplets containing M. tuberculosis released when an infected person coughs, sneezes or talks. |
| HIV/AIDS | Sexual contact, exposure to infected blood (needles, transfusion) and vertical transmission (mother‑to‑child). |
Biological, Social & Economic Factors Influencing Control
| Disease | Biological | Social | Economic |
|---|
| Cholera | Rapid replication in the small intestine; cholera toxin causes profuse watery diarrhoea. | Poor sanitation, lack of safe drinking water, overcrowded urban slums. | Cost of water‑treatment and sewage systems; oral rehydration salts are cheap but need distribution. |
| Malaria | Obligate two‑host life cycle; drug‑resistant strains (e.g., artemisinin‑resistant P. falciparum). | Rural housing without screened windows; night‑time outdoor activities increase exposure. | Funding for insecticide‑treated nets, indoor residual spraying, and antimalarial drugs. |
| Tuberculosis (TB) | Ability to remain latent for years; HIV co‑infection markedly raises susceptibility. | Overcrowded housing, prisons, poor ventilation, stigma limiting health‑seeking. | Long‑term multi‑drug therapy (6–9 months) is expensive; drug‑resistant TB requires costly second‑line drugs. |
| HIV/AIDS | Integration of viral DNA into host genome; high mutation rate → drug resistance. | Stigma, discrimination, risky sexual behaviours, limited awareness of safe practices. | Lifelong antiretroviral therapy (ART) is costly; disparity in access between high‑ and low‑income nations. |
Penicillin – How It Works (and Why It Does Not Work on Viruses)
- Penicillin is a β‑lactam antibiotic; the β‑lactam ring mimics the D‑alanine‑D‑alanine terminus of the peptidoglycan precursor.
- It binds irreversibly to penicillin‑binding proteins (PBPs), the transpeptidases that catalyse cross‑linking of peptidoglycan strands.
- Inhibition of these enzymes prevents formation of the rigid cell wall, leading to osmotic lysis of growing bacteria (especially Gram‑positive).
- Viruses lack a cell wall and do not possess PBPs; therefore penicillin has no target and is ineffective against viral infections.
Antibiotic Resistance – Mechanism, Drivers & Mitigation
What it is
The ability of bacteria to survive and multiply in the presence of an antibiotic that would normally inhibit or kill them.
Key Drivers (syllabus‑specific)
- Over‑use and inappropriate prescribing of antibiotics in human medicine (e.g., for viral infections).
- Use of antibiotics as growth promoters or for disease prevention in agriculture and animal husbandry.
- Incomplete courses of treatment – surviving bacteria may carry resistance genes.
- Horizontal gene transfer (plasmids, transposons) spreading resistance between species.
Consequences
- Emergence of multidrug‑resistant organisms (e.g., MRSA, MDR‑TB).
- Increased morbidity, mortality and health‑care costs.
- Limited treatment options for common infections.
Mitigation Strategies
- Antibiotic stewardship: prescribe the right drug, at the right dose, for the right duration.
- Infection‑control measures in hospitals – hand hygiene, patient isolation, sterilisation of equipment.
- Public education on the dangers of self‑medication and the importance of completing courses.
- Regulation of antibiotic use in agriculture – ban or restrict growth‑promoter applications.
- Research & development of new antimicrobials, vaccines and alternative therapies (e.g., phage therapy).
Why Understanding Transmission Is Crucial
Effective control measures – vaccination, hygiene, quarantine, vector control, safe water provision – are all based on interrupting the specific route by which a pathogen moves between hosts. Accurate knowledge of the transmission pathway for each disease enables targeted, cost‑effective public‑health interventions.
Illustrative Flowchart (suggested)
Pathogen type → Specific disease → Primary transmission route → Key preventive measure(s)
Summary Checklist (exam quick‑ref)
- Infectious diseases are caused by pathogens and are transmissible.
- Pathogen groups: viruses, bacteria, fungi, protozoa, helminths.
- Four syllabus diseases and exact agents:
- Cholera – Vibrio cholerae
- Malaria – Plasmodium falciparum (representative of Plasmodium spp.)
- Tuberculosis – Mycobacterium tuberculosis
- HIV/AIDS – HIV
- Primary transmission routes:
- Cholera – fecal‑oral
- Malaria – vector‑borne (mosquito)
- TB – airborne droplets
- HIV – sexual, blood, vertical
- Key biological features (cholera toxin, malaria blood‑stage, TB latency, HIV integration).
- Biological, social and economic factors influence control of each disease.
- Penicillin kills bacteria by binding PBPs and blocking peptidoglycan cross‑linking; it has no effect on viruses.
- Antibiotic resistance arises from over‑use, agricultural use, incomplete courses, and gene transfer; stewardship, infection control and research are essential.
- Preventive measures must target the specific transmission route of each pathogen.