What you will learn
- classify diseases as infectious or non-infectious and identify the cause of each,
- name the five main types of pathogen and their characteristics,
- describe how pathogens are transmitted and how transmission is controlled,
- distinguish innate and adaptive immunity and explain how vaccination works,
- identify risk factors for common non-infectious diseases.
Worked example 0 Real-world example: a cold vs heart disease
Two adults see a doctor. One has a cold (runny nose, sore throat); the other has high blood pressure and chest pain.
- The cold is caused by a rhinovirus — an infectious pathogen passed by droplets and touch. Control: hand washing, staying home, covering coughs.
- High blood pressure and heart disease are non-infectious. Cause: a mix of genetics, diet, exercise, smoking, and age. Control: medication, exercise, diet change, not quarantine.
Key idea: infectious diseases are caught and spread; non-infectious diseases develop from individual risk factors.
1. Pathogen types
A pathogen is any agent that causes disease.
| Pathogen | Size | Example disease | Key feature |
|---|---|---|---|
| Bacteria | tuberculosis, strep throat, salmonella | single-celled, living, reproduce by binary fission; many treated by antibiotics | |
| Viruses | flu, COVID-19, measles, HIV | not cells; need a host cell to reproduce; treated by antivirals, not antibiotics | |
| Fungi | varies | tinea (athlete’s foot), thrush | multicellular or yeast; grow on skin/mucous surfaces; treated by antifungals |
| Protists | single cells | malaria (Plasmodium), giardia | single-celled eukaryotes; often spread by water or insect vectors |
| Prions | protein | Creutzfeldt-Jakob disease, BSE (“mad cow”) | mis-folded proteins; no DNA or RNA; no cure |
2. Transmission and control
Common transmission modes:
- Droplet / airborne: coughs and sneezes (flu, COVID, TB).
- Direct contact: skin or bodily fluids (chickenpox, STIs).
- Indirect contact: contaminated surfaces (cold, norovirus).
- Food and water: salmonella, cholera, giardia.
- Vector-borne: mosquitos (malaria, dengue), ticks.
- Vertical: parent to offspring (some HIV, rubella).
Control measures match the mode:
| Measure | How it works |
|---|---|
| Hand hygiene | breaks contact transmission |
| Masks, ventilation | reduce droplet transmission |
| Quarantine | separates infected people until no longer contagious |
| Safe water and sanitation | prevents water/food-borne spread |
| Insecticides, bed nets | block vector spread |
| Vaccination | prepares the immune system in advance |
| Public education | encourages all of the above |
3. The immune response
The body has two lines of defence against pathogens.
Innate (non-specific) immunity
Acts against any invader, immediately. It includes:
- Physical barriers: skin, mucous membranes, cilia in airways.
- Chemical defences: stomach acid, lysozyme in tears and saliva.
- Inflammation: blood vessels dilate, bringing white blood cells to the site.
- Phagocytes: white blood cells that engulf and digest pathogens.
Adaptive (specific) immunity
Slower but targeted. Relies on lymphocytes that recognise specific pathogens.
- B-cells produce antibodies — Y-shaped proteins that bind to specific antigens on a pathogen, marking it for destruction.
- T-cells kill infected body cells directly.
- Memory cells remain after an infection; if the same pathogen appears again, the response is much faster and stronger. This is immunological memory.
4. Vaccination
A vaccine contains a harmless version of a pathogen — a dead or weakened form, a fragment, or mRNA instructions to make one protein.
- The vaccine introduces antigens without causing disease.
- B-cells and T-cells mount an adaptive response.
- Memory cells form.
- If the real pathogen later infects the person, memory cells respond quickly — often before symptoms appear.
Herd immunity: when a high proportion of a population is vaccinated, the pathogen cannot spread easily, protecting those who cannot be vaccinated (babies, immune-compromised people).
Worked example 1 Why do we get boosters?
A patient had a tetanus shot 12 years ago. They step on a rusty nail and the doctor recommends a booster. Why?
- Memory cells are long-lived but not permanent; their numbers decline over decades.
- A booster quickly re-stimulates memory cells and raises antibody levels.
- Protection is restored before the tetanus bacterium (Clostridium tetani) can produce dangerous toxin.
Key idea: immunity is not always lifelong — some vaccines need top-ups.
5. Non-infectious diseases
These are not caused by pathogens. Major categories:
| Disease | Main features | Risk factors |
|---|---|---|
| Cardiovascular (heart attack, stroke) | blocked blood vessels | smoking, high blood pressure, high cholesterol, inactivity, age |
| Cancer | uncontrolled cell division | smoking, UV exposure, diet, some viruses (e.g. HPV), genetics |
| Type 2 diabetes | insulin resistance | obesity, inactivity, diet, genetics |
| Asthma | inflamed airways | allergens, pollution, genetics |
| Genetic (e.g. cystic fibrosis) | inherited mutation | family history |
Risk factors are usually a mix of lifestyle, genetic, and environmental factors.
Worked example 2 Risk-factor analysis
A 55-year-old smoker has high blood pressure, is overweight, and has a family history of heart disease. Rank the risk factors as modifiable or non-modifiable and suggest actions.
- Modifiable: smoking (quit), weight (diet, exercise), blood pressure (medication, lifestyle).
- Non-modifiable: age, family history.
- Focus on modifiable factors — even a moderate improvement in each significantly reduces cardiovascular risk.
Key idea: most non-infectious diseases have both genetic and lifestyle components; lifestyle changes can reduce risk even if genes are unfavourable.
Practice: Year 9
Types and terminology
- Define infectious disease and non-infectious disease and give one example of each.
- Name the five main types of pathogen and give one disease caused by each.
- Why do antibiotics not work on viruses?
- Describe two physical and two chemical barriers in innate immunity.
- Distinguish antigen and antibody.
- Define memory cell and explain its role in immunity.
Apply the ideas
- A classmate has chickenpox. List three measures that could reduce the spread to others.
- Explain why vaccination is more effective than waiting for natural infection for diseases like measles.
- In an outbreak of salmonella from a restaurant, public-health officials inspect the kitchen. Why not the staff’s living quarters? Relate to the mode of transmission.
- Why is it important to finish a full course of antibiotics even after you feel better?
- Smoking causes both lung cancer (non-infectious) and increases risk of respiratory infections (infectious). Explain each mechanism briefly.
Data and decisions
- A vaccine gives efficacy. In a community of 10 000 exposed people who are all vaccinated, roughly how many would you expect to still get sick? Why is this important for “herd immunity”?
- Malaria is transmitted by Anopheles mosquitoes. Evaluate three public-health interventions (draining still water, bed nets, indoor insecticide spraying) by linking each to a specific stage in the transmission cycle.
- A 60-year-old has a family history of type 2 diabetes, a BMI of 31, and is sedentary. Suggest three evidence-based actions to lower their diabetes risk.
- Explain why quarantine works for some diseases (measles, COVID) but not for others (heart disease).
Challenge
Harder reasoning
- Antibiotic resistance arises when bacteria with natural variation survive antibiotic treatment and pass on resistance genes. Explain how overuse of antibiotics in humans and livestock accelerates this and propose two control measures.
- Some viruses (influenza) mutate their surface antigens quickly, while others (measles) are much more stable. Relate this to why flu needs a new vaccine every year but measles does not.
- HIV targets T-helper cells of the immune system. Explain why this causes AIDS, and why people with untreated AIDS become vulnerable to infections that rarely harm healthy people.
- Contrast “primary” and “secondary” immune responses in terms of speed, strength, and antibody levels. Sketch a rough graph of antibody concentration over time showing both responses.