1. Overview
An infectious disease is one caused by a pathogen — a disease-causing organism that can be passed from an infected host to others. This makes infectious diseases transmissible, unlike non-infectious conditions such as scurvy or inherited disorders. Four named diseases are studied in detail: cholera (a bacterial gut infection), malaria (a protoctist disease spread by mosquitoes), tuberculosis (TB) (a bacterial lung infection) and HIV/AIDS (a viral disease that attacks the immune system). For each, you need to know the type and name of the pathogen, how it is transmitted, and the biological, social and economic factors that affect how the disease can be prevented and controlled.
Key Definitions
- Infectious disease: a disease caused by a pathogen that can be transmitted from one host to another.
- Pathogen: an organism that causes disease, including some bacteria, viruses, protoctists and fungi.
- Transmission: the passing of a pathogen from an infected host to an uninfected individual.
- Vector: a living organism that carries a pathogen from one host to another without itself developing the disease, such as the female Anopheles mosquito for malaria.
- Contamination: the presence of a pathogen in or on a non-living thing such as water, food or surfaces.
- Endemic disease: a disease that is always present in a population or region at a fairly steady level.
- Antibiotic resistance: the ability of bacteria to survive and grow in the presence of an antibiotic that would normally inhibit or kill them.
- Carrier: an infected individual who shows no symptoms but can still pass the pathogen to others.
- AIDS: the condition that develops when HIV has destroyed so many T-helper lymphocytes that the immune system can no longer defend the body, leaving the person vulnerable to opportunistic infections.
Content
Pathogens and transmissible disease
A pathogen is an organism that causes disease. The main groups are bacteria, viruses, protoctists and fungi. A disease is infectious when its pathogen can move from one host to another; because the pathogen can spread, the disease is also transmissible. This is the key difference from non-infectious diseases (such as a dietary deficiency), which cannot be passed between people.
It is worth being precise about two structures:
- Viruses are not cells. A virus particle has a genome of either DNA or RNA (never both), enclosed in a protective protein coat called a capsid.
- Bacteria are prokaryotic cells with a cell wall, a single circular DNA molecule and ribosomes, but no nucleus.
Knowing these features helps you explain how medicines work. For example, an antibiotic that binds to bacterial ribosomes acts by blocking protein synthesis (translation), while a different antibiotic might instead inhibit cell wall formation. Always match the explanation to the target you are given.
The four named diseases and their pathogens
When writing pathogen names, follow binomial naming rules: capitalise the Genus name, keep the species name lowercase, and italicise (or underline) the whole name.
| Disease | Type of pathogen | Name(s) of pathogen |
|---|---|---|
| Cholera | Bacterium | Vibrio cholerae |
| Malaria | Protoctist | Plasmodium falciparum, P. malariae, P. ovale, P. vivax |
| Tuberculosis (TB) | Bacterium | Mycobacterium tuberculosis, Mycobacterium bovis |
| HIV/AIDS | Virus | human immunodeficiency virus (HIV) |
How each disease is transmitted
Each disease has its own main route of spread. The table below summarises the four; the notes underneath add the key detail for each.
| Disease | Main transmission route | Key detail |
|---|---|---|
| Cholera | Faecal–oral (contaminated water/food) | Spreads where sewage mixes with drinking water |
| Malaria | Vector — female Anopheles mosquito bite | Also via infected blood and across the placenta |
| TB | Airborne droplets (coughing, sneezing) | M. bovis also from unpasteurised milk/cattle |
| HIV | Infected body fluids | Sexual contact, infected blood, mother to child |
Cholera is spread by the faecal–oral route. Vibrio cholerae is carried in the faeces of infected people; when sewage contaminates drinking water or food, anyone who consumes it can be infected. The bacteria multiply in the small intestine and release a toxin that causes severe watery diarrhoea, leading to dehydration. Note the wording: water and food are contaminated (a non-living thing carries the pathogen), whereas a person becomes infected.
Malaria is transmitted by a vector, the female Anopheles mosquito. An infected mosquito injects Plasmodium into the bloodstream when it feeds; a mosquito that later bites an infected person can then transmit the parasite to someone else. Malaria can also pass through infected blood (transfusions or shared needles) and across the placenta from mother to fetus. (You do not need the parasite's life cycle.)
Tuberculosis spreads through the air in tiny droplets. An infected person coughs, sneezes or talks, releasing droplets containing Mycobacterium tuberculosis; others inhale them. Mycobacterium bovis can also be caught from cattle, mainly through unpasteurised milk or close contact. Crowded, poorly ventilated living conditions greatly increase the risk of transmission. TB can remain latent (inactive) for years and become active when the immune system is weakened.
HIV is transmitted through infected body fluids. The main routes are unprotected sexual contact, infected blood (sharing needles or, rarely, transfusions of unscreened blood), and from mother to child across the placenta, during birth, or through breast milk. HIV is not spread by ordinary social contact. It infects and destroys T-helper lymphocytes, gradually weakening the immune system until the person develops AIDS and becomes vulnerable to other infections.
Preventing and controlling the diseases
Effective control combines biological, social and economic measures.
Cholera. Biological/medical: oral rehydration therapy treats sufferers, and vaccines give some protection. Social: providing clean piped water, proper sewage treatment, food hygiene and public education on handwashing breaks the faecal–oral route. Economic: building and maintaining water and sanitation infrastructure is expensive, and outbreaks often follow natural disasters or occur in refugee camps where money and resources are scarce.
Malaria. Biological/medical: antimalarial drugs treat and protect people, insecticide-treated bed nets stop mosquito bites at night, and spraying or draining standing water reduces mosquito breeding. Social: education encourages net use; conflict and population movement spread the disease. Economic: nets, drugs and spraying programmes cost money, and the disease is concentrated in lower-income tropical regions; some mosquitoes have become resistant to insecticides and some Plasmodium strains resistant to drugs, raising costs further.
TB. Biological/medical: the BCG vaccine offers some protection, and a long course of several antibiotics cures most cases — but treatment must be completed to prevent antibiotic resistance developing in the bacteria. Contact tracing identifies and treats people who have been exposed. Social: improving housing and reducing overcrowding lowers transmission; DOTS (directly observed treatment) ensures patients finish their drugs. Economic: the long treatment is costly, and TB is linked to poverty and to HIV co-infection.
HIV. Biological/medical: there is no cure or vaccine, but antiretroviral drugs keep the virus under control, prolong life and reduce transmission; screening blood and giving drugs to pregnant women lowers spread. Social: education promotes condom use and safe needle practice; stigma discourages testing, so reducing it is important. Economic: lifelong drug treatment is expensive and falls hardest on lower-income countries with the highest infection rates.
Worked example
Exam-style question: A coastal town has no piped water supply, and its sewage drains into the same river that residents use for drinking and washing. After heavy flooding, many people develop severe watery diarrhoea. Name the likely disease and its pathogen, and suggest two measures that would reduce future outbreaks. [3]
Model answer:
- The disease is cholera, caused by the bacterium Vibrio cholerae. (The flooding has caused sewage to contaminate the river water, spreading the bacteria by the faecal–oral route.) [1]
- Measure 1: provide a clean, treated piped water supply (and/or treat sewage separately) so drinking water is no longer contaminated with faeces. [1]
- Measure 2: educate residents on hygiene (boiling water, handwashing) and provide oral rehydration therapy to treat cases and limit further spread. [1]
Worked example
Exam-style question: A region introduced a programme giving every household insecticide-treated bed nets in Year 1. The table shows the number of new malaria cases per 1000 people in the following years.
| Year | New malaria cases per 1000 people |
|---|---|
| 0 (before nets) | 240 |
| 1 | 180 |
| 2 | 120 |
| 3 | 95 |
Describe the trend shown and explain, using one biological and one economic point, how the bed-net programme helped control malaria. [4]
Model answer:
- Trend: the number of new cases fell after the nets were introduced, dropping from 240 to 95 per 1000 people over three years — a decrease of 145 per 1000 (about a 60% fall). The decline was steepest in the first two years, then began to slow. [1 for trend + 1 for supporting data]
- Biological point: the insecticide-treated nets act as a physical barrier at night and kill mosquitoes that land on them, so fewer people are bitten by the female Anopheles vector and transmission of Plasmodium is reduced. [1]
- Economic point: preventing cases lowers the cost of treatment and lost working days, and distributing nets is cheaper than repeatedly treating sick people, so the programme is cost-effective in a lower-income region. [1]
Key Equations
This is a qualitative topic; there are no equations to learn.
Common Mistakes to Avoid
- Saying a virus contains both DNA and RNA, or that the capsid is made of carbohydrate. A virus particle has either DNA or RNA as its genome, enclosed in a protein coat (capsid).
- Explaining an antibiotic's action with the wrong mechanism. Match the explanation to the target you are given: an antibiotic that binds to ribosomes blocks protein synthesis, whereas one that targets the cell wall prevents the wall forming. Do not use the cell-wall explanation for a ribosome-targeting drug.
- Confusing immunity with resistance. Immunity is a host organism's own defence against a pathogen; resistance is a pathogen's ability to survive a drug. People develop immunity; bacteria develop antibiotic resistance.
- Saying people become "immune to antibiotics". Antibiotics act on bacteria, not on human cells, so only the bacteria can become resistant. Humans develop immunity to pathogens, not to antibiotics.
- Describing water or food as "infected". Use contaminated for non-living things such as water, food or surfaces, and infected only for living hosts such as people.
- Writing pathogen names incorrectly. Always capitalise the Genus and use lowercase for the species, e.g. Mycobacterium tuberculosis, Vibrio cholerae — and italicise or underline them.
- Listing only medical control measures. "Prevention and control" questions reward biological, social and economic points — include clean water, education, housing, stigma and cost, not just drugs and vaccines.
Exam Tips
- Learn the pathogen type and exact name for all four diseases as a quick recall table — these are common one-mark questions and the spelling and capitalisation must be right.
- For "how is it transmitted" answers, name the specific route (faecal–oral, vector/mosquito bite, airborne droplets, infected body fluids) rather than a vague "spread by contact".
- In control questions, label your points as biological, social or economic to make sure you cover the full range the question is testing.
- When a question says "discuss" the biological, social and economic factors, do not just list points across the three categories — for the top marks weigh them against each other, e.g. note that a measure can be biologically effective but economically unaffordable in a lower-income region.
- Use the words contaminated (non-living things) and infected (living hosts) correctly — precise terminology earns easy marks.
- When explaining how a drug works, use the information given in the question (e.g. its target) rather than assuming all antibiotics act in the same way.
- For data questions, always describe the trend and quote figures from the table to support it, then link the change to a named biological, social or economic factor.
- Remember the distinction between immunity (the host) and resistance (the pathogen) — mixing these up loses easy marks.