Communicable Disease
What is a Communicable Disease?
A communicable disease (infectious disease) is one that can be spread from person to person (or from animals to humans). It is caused by a pathogen — a microorganism that enters the body and causes harm.
Types of Pathogen
| Pathogen | Type of organism | Examples | Treatment |
|---|---|---|---|
| Bacteria | Prokaryote | Salmonella (food poisoning), Streptococcus (strep throat), TB (Mycobacterium tuberculosis) | Antibiotics |
| Virus | Non-living particle; uses host cells | Influenza, COVID-19, HIV, measles | Antivirals (limited); vaccination |
| Fungi | Eukaryote | Athlete's foot (Tinea pedis), ringworm | Antifungal drugs |
| Protists | Single-celled eukaryote | Malaria (Plasmodium) | Antimalarial drugs (e.g., artemisinin) |
How pathogens cause disease: By damaging cells/tissues and/or producing toxins.
How viruses replicate: They invade host cells, take over the cell's machinery, reproduce rapidly, and burst the cell — releasing new viruses to infect more cells.
Spread of Disease
Communicable diseases spread by:
- Direct contact — touching infected person or contaminated surfaces
- Droplet infection — coughing/sneezing releases pathogens in tiny droplets (influenza, COVID-19)
- Contaminated water or food — Salmonella, cholera
- Vectors — organisms that carry pathogens to humans; mosquitoes carry Plasmodium for malaria
- Sexual contact / exchange of bodily fluids — HIV
The Body's Defences
Non-Specific Defences (Physical and Chemical Barriers)
These act against all pathogens — they don't need to recognise a specific pathogen.
- Skin — physical barrier; sebaceous glands produce sebum (mildly acidic) to kill bacteria
- Mucus and cilia in the airways — mucus traps pathogens; cilia waft them to the throat to be swallowed
- Stomach acid (HCl) — pH ~2; kills most pathogens in food and drink
- Tears and saliva — contain lysozyme enzyme that breaks down bacterial cell walls
Specific Immune Response
White blood cells (leukocytes) provide the specific immune response:
- Phagocytes (e.g., neutrophils, macrophages) — engulf and digest pathogens (phagocytosis). Non-specific.
- Lymphocytes — recognise specific antigens on pathogens:
- B-lymphocytes: produce antibodies (Y-shaped proteins that bind to specific antigens). Antibodies can neutralise pathogens, flag them for phagocytosis, or clump pathogens together (agglutination).
- T-lymphocytes: help coordinate immune response; some destroy infected cells.
- Memory cells — produced after first exposure; remain in the blood long-term. On re-exposure, memory cells produce a faster and larger antibody response — the person is now immune.
Vaccination
Vaccines introduce a harmless form of the pathogen (dead/weakened pathogen, antigen or mRNA) to trigger the immune response without causing disease. Memory cells are produced — the person is immune for future real infections.
Herd immunity: If enough of a population is vaccinated, the pathogen cannot spread easily → even unvaccinated individuals are protected.
WJEC context: Wales runs its own immunisation programmes; WJEC papers often include data on vaccination uptake and disease incidence.
Antibiotics
Antibiotics (e.g., penicillin, amoxicillin) kill or inhibit the growth of bacteria by disrupting bacterial cell wall synthesis, protein synthesis, etc.
Why antibiotics don't work on viruses: Viruses lack cell walls and use the host cell's own machinery — antibiotics target bacterial structures not present in viruses.
Antibiotic resistance: Natural selection means bacteria with mutations that resist antibiotics survive and reproduce → resistant strain spreads. To slow resistance:
- Complete the full antibiotic course
- Don't prescribe antibiotics for viral infections
- Develop new antibiotics
MRSA (methicillin-resistant Staphylococcus aureus) is a well-known antibiotic-resistant "superbug" often tested on WJEC papers.
AI-generated · claude-opus-4-7 · v3-wjec-combined-science