Communicable (infectious) diseases — AQA GCSE Combined Science
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Communicable (infectious) diseases explained
This statement requires you to explain transmission routes for diseases caused by four pathogen groups: viruses, bacteria, protists and fungi, in animals and plants.
Read the full explanation
For animals, examples include influenza virus spread by airborne droplets, Salmonella bacteria spread by contaminated food, the protist Plasmodium spread by mosquito vectors, and the fungus Trichophyton spread by direct contact. For plants, examples include tobacco mosaic virus spread by direct contact and contaminated tools, crown gall bacteria spread through wounds, potato blight protist spread by spores in wind and water, and rose black spot fungus spread by spores in wind and water. Explaining spread means naming the pathogen group, the route and how the route lets the pathogen reach a new host.
Students should be able to explain how the spread of diseases can be reduced or prevented.
Spread of disease depends on a pathogen leaving one host, travelling, and entering a new host. Breaking any link reduces transmission. Direct contact is limited by hand-washing, covering cuts, using condoms and isolating infected people. Water-borne spread is reduced by clean water supplies, sewage treatment and chlorination. Airborne spread is reduced by ventilation, covering the mouth when coughing and using masks. Food-borne spread is reduced by refrigeration, cooking thoroughly and hygiene. Vectors such as mosquitoes are controlled by nets, insecticides and draining standing water. Vaccination makes hosts resistant, so the pathogen cannot multiply and pass on. In an exam, name the transmission route, then match a control method to that route and explain why it works.
Pathogens are microorganisms that cause infectious disease. Pathogens may be viruses, bacteria, protists or fungi. They may infect plants or animals and can be spread by direct contact, by water or by air.
A pathogen is a microorganism that causes infectious disease. The four main groups are viruses, bacteria, protists and fungi. Viruses reproduce inside host cells and damage them; bacteria multiply in tissues and may release toxins; protists are often single-celled and may be carried by vectors; fungi can grow on surfaces and penetrate tissues. Pathogens infect both plants and animals, for example tobacco mosaic virus in plants and influenza virus in humans. Spread occurs by direct contact, by water or by air. Direct contact includes touching, body fluids and contaminated surfaces. Water-borne spread occurs when pathogens enter drinking water or are carried in water droplets. Airborne spread occurs in droplets and aerosols released by coughing or sneezing.
Bacteria and viruses may reproduce rapidly inside the body.
Pathogens cause illness only if they can multiply within the host. Bacteria are single-celled prokaryotes that divide by binary fission: one cell splits into two, so numbers rise geometrically, for example 1 → 2 → 4 → 8, doubling each generation. Viruses are much smaller and are not cells; they must enter a host cell and use its machinery to make many identical copies of their genetic material and protein coat, then burst out to infect more cells. Because both processes are fast and the body is warm and nutrient-rich, a small initial dose can become a huge population quickly, producing symptoms before the immune system controls the infection.
Bacteria may produce poisons (toxins) that damage tissues and make us feel ill.
Many disease-causing bacteria harm the body not only by multiplying but by releasing toxins. These toxins are substances produced by the bacteria that interfere with host cells, damage tissues and trigger symptoms such as fever, vomiting or diarrhoea. For example, in food poisoning, bacteria such as Salmonella release toxins in the gut, irritating the lining and causing illness. The toxin may act near the site of infection or be carried in the blood to affect other organs. This explains why symptoms can appear even when the bacteria themselves remain in one place, and why treating the effects of toxins, as well as the infection, matters.
Viruses live and reproduce inside cells, causing cell damage.
Viruses are not cells and cannot carry out life processes independently, so they are obligate parasites. A virus particle attaches to a host cell, injects its genetic material, and uses the host's ribosomes and enzymes to make new virus particles. Because replication happens inside the cell, the host cell's normal chemistry is disrupted and the cell may burst, releasing many new viruses. This explains why viral infections damage tissues and cause symptoms. For example, influenza virus infects cells lining the airways; the resulting cell damage contributes to fever and coughing. Antibiotics do not kill viruses because viruses lack bacterial targets such as cell walls.
Your focus
- Name the four pathogen groups and give an example disease for each.
- Explain the main transmission routes for viral, bacterial, protist and fungal diseases in animals.
- Explain the main transmission routes for viral, bacterial, protist and fungal diseases in plants.
Show all 18 objectives
- Describe how direct contact, water, air, food and vectors spread pathogens.
- Explain how named control methods interrupt specific transmission routes.
- Apply the chain-of-infection model to suggest how an outbreak could be reduced.
- Define pathogen and name the four main pathogen groups.
- Give examples of pathogens that infect plants and animals.
- Describe direct contact, water and air as routes of pathogen spread.
- Describe how bacteria reproduce by binary fission inside the body.
- Explain why viruses must use a host cell to replicate.
- Relate rapid pathogen reproduction to the development of symptoms.
- Describe how bacterial toxins damage tissues and cause illness.
- Explain why symptoms can occur away from the site of a bacterial infection.
- Distinguish between harm caused by toxins and harm caused by bacterial growth.
- State that viruses reproduce inside living cells.
- Describe how viral replication damages host cells.
- Explain why viral infections cause symptoms and why antibiotics are ineffective.
Communicable (infectious) diseases exam tips
Marking Points
- Identify viruses, bacteria, protists and fungi as the four pathogen groups named in the statement.
- Explain that viruses spread in animals through airborne droplets, direct contact or contaminated body fluids, for example influenza.
- Explain that bacteria spread in animals through contaminated food or water, direct contact or droplets, for example Salmonella.
- Explain that protists spread in animals through vectors such as mosquitoes, for example Plasmodium causing malaria.
- Explain that fungi spread in animals through direct contact or spores, for example athlete's foot.
- Explain that plant viruses spread through direct contact or contaminated tools, for example tobacco mosaic virus.
- Explain that plant bacteria spread through wounds, water or vectors, for example crown gall disease.
- Explain that plant protists and fungi spread through spores carried by wind or water, for example potato blight and rose black spot.
- Identify the transmission route first: direct contact, water, air, food or a vector, because the control method must interrupt that specific route.
- Hand-washing, disinfecting surfaces and covering cuts reduce direct contact and fomite transmission by removing or containing pathogens.
- Clean water, sewage treatment and chlorination reduce water-borne diseases such as cholera by killing or removing pathogens before ingestion.
- Ventilation, covering the mouth when coughing and masks reduce airborne droplet transmission by diluting or trapping droplets.
- Refrigeration, thorough cooking and hygienic food handling reduce food-borne illness by slowing or killing pathogen growth.
- Vaccination and isolating infected individuals reduce spread by making hosts resistant or preventing pathogen release to new hosts.
- Vector control, such as mosquito nets, insecticides and draining standing water, reduces diseases spread by insects.
- Define a pathogen as a microorganism that causes infectious disease, distinguishing it from non-infectious causes such as diet or genetics.
- Name the four pathogen groups: viruses, bacteria, protists and fungi, and give one example of each.
- State that pathogens can infect both plants and animals, with named examples such as tobacco mosaic virus in plants and influenza virus in humans.
- Describe direct contact spread, including touching, body fluids and contaminated surfaces.
- Describe water-borne spread, including drinking contaminated water and water carrying pathogens.
- Describe airborne spread, including droplets and aerosols released by coughing or sneezing.
- Bacteria reproduce by binary fission, in which one parent cell divides into two genetically identical daughter cells.
- Bacterial population growth is rapid and can be exponential when conditions inside the body are favourable.
- Viruses do not divide by binary fission; they replicate only inside a host cell using the host's enzymes and ribosomes.
- Viral replication involves copying the viral genetic material and making viral protein coats, then releasing new virus particles.
- Rapid reproduction inside the body means pathogen numbers can rise before the immune system removes them, so symptoms develop.
- The body provides warmth, moisture and nutrients that support fast pathogen reproduction.
- Some pathogenic bacteria release toxins as they grow and reproduce in the body.
- Toxins are poisons that damage host tissues and disrupt normal cell or organ function.
- Toxin damage produces symptoms such as fever, pain, vomiting or diarrhoea, making the person feel ill.
- Toxins may act locally at the site of infection or spread in the blood to affect other parts of the body.
- The harm caused by a bacterial infection can be due to toxins as well as to direct damage by the bacteria.
- Viruses are not cells and are much smaller than bacteria.
- Viruses reproduce only inside living host cells, using the host cell's machinery.
- Viral genetic material enters the host cell and directs synthesis of new virus particles.
- Replication inside the cell disrupts normal cell activity and damages or destroys the cell.
- Cell damage and the release of new virus particles produce the symptoms of viral disease.
- Antibiotics are ineffective against viruses because viruses do not have bacterial cell structures.
Examiner Tips
- 💡Link each named disease to its pathogen group and its transmission route in one clear sentence.
- 💡Use comparative language when asked about more than one pathogen group, for example 'whereas fungi spread by spores, protists may be spread by vectors'.
- 💡Include a plant example as well as an animal example when the question asks about both, so both parts of the statement are covered.
- 💡Use the chain-of-infection idea: pathogen leaves host, travels, enters new host; then explain which link your method breaks.
- 💡Give a named disease and its route where possible, for example cholera via water or influenza via air, to make the explanation concrete.
- 💡For 'explain' questions, write a because clause linking the method to reduced transmission, not just a list of actions.
- 💡Learn one named example for each pathogen group so you can illustrate the definition quickly.
- 💡When asked how a disease spreads, name the route and give a concrete situation, such as droplets from a cough entering the air.
- 💡Use precise terms: infectious disease, pathogen, host, vector and transmission, rather than vague words like 'germs'.
- 💡Use the term binary fission when describing bacterial reproduction, and state that it produces identical cells.
- 💡For viruses, always mention a host cell, because replication depends on the host's machinery.
- 💡Link rapid reproduction to symptom timing: more pathogens means more damage and a greater chance of feeling ill.
- 💡State clearly that toxins are poisons produced by bacteria, not the bacteria themselves.
- 💡Give a named example, such as food poisoning, to show how toxins cause symptoms.
- 💡Link toxin action to tissue damage and to feeling ill, using both ideas in your answer.
- 💡Use the phrase 'inside cells' when explaining viral reproduction, because the specification links reproduction to cell damage.
- 💡Link each stage to the consequence for the host cell, for example entry, replication, then cell damage.
- 💡If asked why antibiotics do not work, compare viruses with bacteria rather than simply saying 'viruses are different'.
Common Mistakes
- Treating all pathogens as spreading in the same way; correction: each pathogen group has characteristic routes, such as vectors for Plasmodium and spores for fungi.
- Confusing a vector with the pathogen itself; correction: a vector is an organism that carries and transmits the pathogen, such as a mosquito carrying Plasmodium.
- Assuming tobacco mosaic virus is spread by vectors like aphids; correction: TMV is spread by direct contact between plants and via contaminated tools.
- Saying 'kill germs' without naming the route or method; correction: state the route and the specific control, for example chlorination kills water-borne pathogens.
- Confusing a treatment with prevention; correction: antibiotics treat bacterial infection but do not prevent spread, whereas vaccination prevents infection.
- Assuming all diseases spread the same way; correction: match each control to its route, such as masks for airborne spread and clean water for water-borne spread.
- Calling all microorganisms pathogens; correction: most microorganisms are harmless or useful, and only some cause disease.
- Saying bacteria are viruses or treating the groups as identical; correction: viruses reproduce inside host cells, bacteria multiply outside cells and may release toxins.
- Forgetting that plants get diseases too; correction: include a plant example such as tobacco mosaic virus alongside an animal example.
- Saying viruses reproduce by binary fission like bacteria; correction: viruses replicate inside host cells and cannot reproduce independently.
- Describing bacteria as 'growing bigger' rather than increasing in number; correction: reproduction increases cell number by division.
- Assuming all bacteria are harmful; correction: many bacteria are harmless or useful, and only some are pathogens.
- Confusing toxins with the bacteria themselves; correction: toxins are poisonous substances produced by the bacteria.
- Thinking all bacteria produce toxins; correction: only some pathogenic bacteria do, and harmless bacteria do not.
- Assuming toxins only affect the site of infection; correction: toxins can be transported in the blood and affect distant tissues.
- Saying viruses are a type of bacterium: correct this by stating viruses are non-cellular and much smaller, with a different structure.
- Claiming viruses reproduce on their own: correct this by explaining they need a living host cell and its enzymes and ribosomes.
- Confusing 'live' with independent life: correct this by describing viruses as obligate parasites that can only replicate inside cells.