Communicable Diseases and Pathogens
Communicable diseases are diseases that can be passed from one organism to another, caused by pathogens: bacteria, viruses, protists and fungi. Bacteria reproduce rapidly inside the body and may produce toxins that damage tissues, while viruses live and reproduce inside cells, usually bursting and damaging the cell when new viruses are released, and pathogens spread mainly by direct contact, water and air, with vectors such as mosquitoes carrying some between hosts.
Before you start
No specific prerequisites - this is a good place to start.
Method
- Sort every named disease into its pathogen type first, because the exam usually asks for that: viral (measles, HIV, tobacco mosaic virus), bacterial (salmonella, gonorrhoea), fungal (rose black spot), protist (malaria).
- For each disease, learn four things in the same order: pathogen type, main symptoms, how it spreads, and how it is controlled or prevented. Answers gain marks for the mechanism, not just the label.
- Explain how bacteria and viruses make you ill differently: bacteria reproduce rapidly and produce toxins that damage cells and tissues; viruses reproduce inside body cells, and the cells burst or are damaged when the new virus particles are released.
- Match the prevention to the route of transmission: hygiene and hand washing for direct contact, condoms and antibiotic treatment for gonorrhoea, destroying vectors or using mosquito nets for malaria, isolating infected individuals for airborne disease, and vaccination where one exists.
- For plant diseases, learn the visible signs and the effect on the plant: tobacco mosaic virus gives a distinctive mosaic pattern of discolouration, which reduces the leaf area available for photosynthesis and so reduces growth; rose black spot causes purple or black spots on leaves, which then turn yellow and drop, again reducing photosynthesis.
- For data questions on the spread of disease, describe the trend using figures from the graph, then link it to the mechanism (for example a fall after a vaccination programme is introduced), and be careful to say a correlation is consistent with a cause rather than proving it.
Worked example
In a population of 2000 people, 150 catch measles in a year. In the following year a vaccination programme reaches 90 percent of the population, and 12 people catch measles. Calculate the percentage of the population infected in each year and the percentage decrease in cases.
- Year 1: percentage infected = (150 / 2000) x 100 = 7.5 percent.
- Year 2: percentage infected = (12 / 2000) x 100 = 0.6 percent.
- Find the decrease in cases: 150 - 12 = 138 fewer cases.
- Percentage decrease = (138 / 150) x 100 = 92 percent.
- Interpret with biology: vaccinating a large proportion of the population means the pathogen cannot spread easily, because most people it reaches are immune, so even unvaccinated people are less likely to catch it.
- Add the caution the mark scheme rewards: the fall is consistent with the vaccination programme causing it, but other factors, such as improved hygiene or a change in reporting, have not been ruled out.
Practice questions
Try each question, then tap to reveal the answer.
Q1Define a communicable disease.Show answer
Answer: A disease caused by a pathogen that can be passed from one organism to another.
Q2Name the type of pathogen that causes malaria.Show answer
Answer: A protist, spread by mosquitoes acting as vectors.
Q3How do bacteria make a person feel ill?Show answer
Answer: They reproduce rapidly and produce toxins that damage cells and tissues.
Q4State two ways measles is prevented from spreading.Show answer
Answer: Vaccination of young children, and isolating infected people, since it spreads by droplets from sneezing and coughing.
Q5Describe the symptoms of rose black spot and its effect on the plant.Show answer
Answer: Purple or black spots develop on leaves, which then turn yellow and drop early. With less leaf area, photosynthesis is reduced and growth slows.
Q6Why can HIV not be treated with antibiotics?Show answer
Answer: Antibiotics kill bacteria; HIV is a virus, and viruses reproduce inside body cells, so drugs that damage them tend to damage the body cells too. HIV is controlled with antiretroviral drugs.
Q7Give one way of reducing the spread of malaria.Show answer
Answer: Prevent the vector from breeding by draining standing water, or stop it biting people using insecticide-treated mosquito nets.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Explain how salmonella food poisoning is caused and describe two ways it is prevented in the UK.
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A gardener notices a mosaic pattern of discolouration on the leaves of tomato plants and the plants are growing poorly. Explain the link between the two observations.
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Compare how a viral disease and a bacterial disease cause illness, and explain why this difference matters when choosing a treatment.
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See real GCSE Science past-paper questions, with official mark schemes →
Free printable worksheet
Want more practice on paper? Download the communicable diseases and pathogens worksheet pack - 20 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 17 of GCSE Biology Workbook, the whole course as one free printable PDF.
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