Communicable (infectious) diseases — AQA GCSE Biology
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Communicable (infectious) diseases explained
Communicable diseases are caused by pathogens and can pass from one organism to another.
Read the full explanation
Viruses spread in animals through droplets, direct contact, contaminated food or water, and blood or body fluids; in plants they spread through vectors such as aphids and by cuttings or grafting. Bacteria spread in animals through contaminated food and water, direct contact and droplets; in plants through wounds, water and vectors. Protists are often spread by vectors, such as mosquitoes carrying malaria, or by water; in plants they may enter through roots or wounds. Fungi spread by spores carried in air, water or on surfaces, and by direct contact; in plants fungal spores often enter through leaves or wounds. A useful method is to name the pathogen type, the route and one control measure.
Students should be able to explain how the spread of diseases can be reduced or prevented.
The spread of communicable disease can be reduced or prevented by interrupting the route by which pathogens travel. Good hygiene, such as hand washing and safe food preparation, removes pathogens and prevents contamination. Destroying or controlling vectors, for example using mosquito nets and insecticides, stops pathogens reaching new hosts. Vaccination gives immunity so that individuals do not become hosts who pass pathogens on. Isolating infected individuals, quarantining arrivals and using sterile equipment reduce direct and indirect contact. In plants, removing infected material, controlling aphids and using clean tools or disease-resistant varieties limit spread. A useful method is to name the route, then choose a control that blocks that route.
Pathogens are microorganisms that cause infectious disease.
A pathogen is any microorganism that causes disease when it enters a host. The key idea is causation: the microorganism itself, not a toxin in food or a genetic fault, triggers the illness, and the disease can pass from one organism to another. Many microorganisms are harmless or useful, such as gut bacteria and decomposers, so pathogen is a functional label rather than a whole group. For example, the bacterium that causes tuberculosis is a pathogen, whereas the bacteria making yoghurt are not. Students should link the term to transmission, since infectious means the disease can spread between hosts. Recognising a named microorganism as a pathogen therefore requires stating both that it is a microorganism and that it causes disease.
Pathogens may be viruses, bacteria, protists or fungi.
Pathogens belong to four broad groups: viruses, bacteria, protists and fungi. Each group has a different cell structure and method of causing harm, so treatment differs. Viruses are not cells; they reproduce inside host cells and destroy them. Bacteria are single-celled organisms that can divide rapidly and may release toxins. Protists are mostly single-celled eukaryotes, and some are parasites transmitted by vectors. Fungi may be single-celled or multicellular and can grow on and penetrate host tissue. Examples include influenza virus, the bacterium causing cholera, the protist causing malaria and the fungus causing rose black spot. Students should be able to classify a named pathogen into its group and use that group to predict how it harms the host.
They may infect plants or animals and can be spread by direct contact, by water or by air.
Pathogens are not restricted to humans: they infect plants and other animals too, impacting food security and ecosystems. Transmission routes include direct contact, water and air. Direct contact covers touching an infected organism, contact with body fluids, or transfer between plant leaves. Water-borne spread occurs when pathogens contaminate drinking water, as with cholera. Airborne spread occurs when pathogens are carried in droplets or aerosols breathed out by an infected host, as with influenza. For plants, vectors such as insects also matter, but the specification examples are direct contact, water and air. Students should match a named disease to its likely route and explain how that route allows the pathogen to reach a new host.
Bacteria and viruses may reproduce rapidly inside the body.
Once inside a host, bacteria and viruses increase in number quickly, and this rapid reproduction explains why symptoms appear soon after infection. Bacteria are living cells that divide by binary fission, so one cell can become two, then four, then eight, giving exponential growth when conditions are favourable. Viruses are not cells; they enter host cells and use the host's machinery to make many copies of themselves, then burst out and infect more cells. The resulting damage and the host immune response produce symptoms. Students should describe the difference in mechanism, not just the shared outcome of rapid increase, and should link the increase to harm such as cell destruction or toxin release.
Bacteria may produce poisons (toxins) that damage tissues and make us feel ill.
Bacteria are single-celled prokaryotes that cause communicable disease. While some directly invade host cells, many cause harm by releasing toxins — poisonous chemicals that damage tissues and produce symptoms like fever or diarrhoea. For example, in food poisoning, Salmonella multiply in the gut and release toxins that irritate the intestinal lining. The toxin is the harmful product; the feeling of illness is the body's response to tissue damage. This explains why antibiotics, which kill bacteria or inhibit their growth, can treat bacterial infections but are ineffective against viruses. Understanding toxins explains why some bacterial diseases are dangerous even when the bacterial cells remain outside host cells.
Viruses live and reproduce inside cells, causing cell damage.
Viruses are much smaller than bacteria and are not cells. They are obligate parasites: they can only reproduce inside a host cell. A virus attaches to a specific host cell, injects its genetic material, and uses the cell's machinery to make copies of its own genetic material and protein coat. These copies assemble into new virus particles. The host cell is often damaged or destroyed when the new viruses burst out, and this cell damage produces the symptoms of the viral disease. For example, influenza viruses infect cells lining the respiratory tract, damaging them and causing fever, cough and aching. Because viruses rely on host cells, antibiotics that target bacterial structures do not work against them. This explains why viral diseases such as measles and HIV are difficult to treat directly and why vaccination is so important.
Your focus
- Identify viruses, bacteria, protists and fungi as types of pathogen.
- Explain at least one route by which each pathogen type spreads in animals and in plants.
- Link a named route of spread to a suitable measure that reduces transmission.
Show all 24 objectives
- Describe measures that reduce or prevent the spread of communicable disease.
- Explain how a named measure interrupts a specific route of pathogen transmission.
- Apply prevention methods to both animal and plant disease examples.
- State that a pathogen is a microorganism causing infectious disease.
- Give a named example of a pathogen and the disease it causes.
- Distinguish pathogens from harmless microorganisms in a given list.
- Name the four groups of pathogen.
- Match a named pathogen to its group.
- Describe one structural or reproductive feature of each group.
- State that pathogens infect plants as well as animals.
- Identify direct contact, water and air as transmission routes.
- Match a named disease to its transmission route and explain the link.
- Describe how bacteria reproduce inside the body.
- Describe how viruses replicate inside host cells.
- Explain how rapid reproduction leads to disease symptoms.
- State that some bacteria produce toxins that damage tissues.
- Explain how toxins lead to symptoms that make a person feel ill.
- Distinguish bacterial toxin damage from direct cell invasion by pathogens.
- Describe how viruses reproduce inside host cells.
- Explain how viral reproduction causes cell damage and symptoms.
- Compare viruses with bacteria in terms of structure and treatment.
Communicable (infectious) diseases exam tips
Marking Points
- Viruses spread in animals by droplets, direct contact, contaminated food or water, and blood or body fluids; in plants by vectors such as aphids and by cuttings or grafting.
- Bacteria spread in animals by contaminated food and water, direct contact and droplets; in plants through wounds, water and vectors.
- Protists spread mainly by vectors such as mosquitoes, and by contaminated water; in plants they can enter through roots or wounds.
- Fungi spread by spores carried in air, water or on surfaces, and by direct contact; in plants spores often enter through leaves or wounds.
- Each explanation should link a named pathogen type to a specific route of spread in either an animal or a plant.
- Control measures such as hygiene, vector control and quarantine can be linked to interrupting the named route.
- Hygiene measures such as hand washing, disinfecting surfaces and safe food preparation reduce pathogen transfer.
- Vector control, including mosquito nets, insecticides and removing standing water, interrupts transmission by vectors.
- Vaccination protects individuals and can reduce the number of hosts available to pass pathogens on.
- Isolation, quarantine and sterile equipment reduce direct and indirect contact between infected and healthy individuals.
- In plants, removing infected material, controlling aphids and using clean tools or resistant varieties reduce spread.
- Each control should be linked to the specific route of spread it interrupts.
- Defines a pathogen as a microorganism that causes disease.
- States that the disease caused is infectious, meaning it can be transmitted between organisms.
- Distinguishes pathogens from harmless or beneficial microorganisms such as decomposers and gut bacteria.
- Links a named example, such as the bacterium causing tuberculosis or a virus causing influenza, to disease causation.
- Uses the term correctly when describing how a disease spreads from host to host.
- Lists viruses, bacteria, protists and fungi as the four pathogen groups.
- Describes viruses as non-cellular agents that reproduce inside host cells.
- Describes bacteria as single-celled organisms that divide rapidly and may release toxins.
- Describes protists as eukaryotic microorganisms, some of which are parasites spread by vectors.
- Describes fungi as organisms that can grow on and penetrate host tissue, causing damage.
- Classifies a named pathogen correctly into one of the four groups.
- States that pathogens can infect both plants and animals.
- Describes direct contact as transfer by touching an infected organism or its body fluids.
- Describes water-borne transmission through contaminated water supplies.
- Describes airborne transmission through droplets or aerosols inhaled by a new host.
- Links a named disease, such as cholera to water or influenza to air, to its main transmission route.
- Explains how a transmission route allows a pathogen to reach and enter a new host.
- States that bacteria and viruses reproduce rapidly once inside the body.
- Describes bacterial reproduction as cell division, producing genetically identical cells.
- Describes viral reproduction as replication inside host cells using host cell machinery.
- Links rapid reproduction to damage, such as cell destruction or toxin release.
- Explains that rapid increase in pathogen numbers leads to symptoms of disease.
- Bacteria are prokaryotic microorganisms that can cause communicable diseases.
- Some bacteria release toxins (poisons) as metabolic products.
- Toxins damage tissues, for example the lining of the gut or respiratory tract.
- Tissue damage produces symptoms such as fever, pain, vomiting or diarrhoea, making the person feel ill.
- The harm can occur even when bacteria remain outside host cells, because the toxin travels to and affects tissues.
- Antibiotics can kill bacteria and stop toxin production, unlike treatments for viral infections.
- Viruses are not cells and are much smaller than bacteria.
- Viruses are obligate parasites that can only reproduce inside a host cell.
- The virus uses the host cell's machinery to replicate its genetic material and make new virus particles.
- New virus particles are released, often destroying or damaging the host cell.
- Cell damage caused by viral replication produces the symptoms of the disease.
- Antibiotics do not kill viruses because viruses lack the bacterial structures that antibiotics target.
Examiner Tips
- 💡Structure answers as pathogen type, route of spread and one control measure to cover the explanation fully.
- 💡Use named examples such as influenza virus, Salmonella bacteria, malaria protist and athlete's foot fungus to make routes concrete.
- 💡For plant diseases, mention vectors such as aphids and entry through wounds or leaves rather than repeating animal routes.
- 💡For each measure, state the route it blocks, such as 'mosquito nets prevent mosquitoes biting and passing on the malaria protist'.
- 💡Use plant and animal examples to show the full range of prevention methods.
- 💡Avoid absolute claims such as 'all diseases can be prevented'; describe reduction or prevention for named examples.
- 💡Use the full phrase microorganism that causes disease in definitions to secure the causation idea.
- 💡When naming an example, state the disease as well as the pathogen so the link is explicit.
- 💡Check whether a question asks for a definition or an example, and answer the precise command word.
- 💡Learn one named example for each of the four groups so classification questions are quick.
- 💡Use structural features, such as presence of a cell or mode of reproduction, to justify the group.
- 💡If asked to compare groups, give one similarity and one difference rather than two separate descriptions.
- 💡Use the phrase direct contact, water or air exactly when listing routes to match the specification.
- 💡For each route, add one sentence explaining how the pathogen reaches a new host.
- 💡Remember that cholera is spread by contaminated water, whereas droplet infection is an example of airborne transmission.
- 💡Contrast bacterial division with viral replication in one sentence to show understanding.
- 💡Use the term host cell when describing viral reproduction.
- 💡Link rapid reproduction to symptom onset so the explanation answers why the disease develops quickly.
- 💡Use the word toxin and link it explicitly to tissue damage and symptoms in the same sentence.
- 💡Give a named example such as food poisoning caused by Salmonella to show application, not just recall.
- 💡When comparing with viruses, state clearly that antibiotics target specific bacterial structures or processes, such as cell walls. Viruses are acellular and lack these structures, and they reproduce inside host cells, which is why antibiotics cannot destroy them.
- 💡Use the phrase host cell and state that reproduction occurs inside it.
- 💡Link cell damage directly to symptoms to show cause and effect.
- 💡Contrast viruses with bacteria in one sentence to demonstrate understanding of why treatments differ.
Common Mistakes
- Treating all pathogens as spreading in the same way: viruses, bacteria, protists and fungi have different routes and vectors.
- Confusing a vector with a pathogen: a vector such as a mosquito carries a pathogen but is not itself the disease-causing organism.
- Giving only a disease name without naming the pathogen type or route of spread, which does not explain transmission.
- Listing control measures without linking each one to a route of spread, which does not explain how spread is reduced.
- Claiming that antibiotics prevent viral diseases: antibiotics act on bacteria and do not treat or prevent viral infections.
- Assuming vaccination gives immediate protection: immunity develops after the immune response, so it is not instantaneous.
- Calling every microorganism a pathogen; correct by stating that only those causing disease are pathogens.
- Confusing infectious with inherited or deficiency disease; correct by linking infectious disease to transmission by a pathogen.
- Saying a pathogen is a poison rather than an organism; correct by describing it as a microorganism that causes disease.
- Treating viruses as cells; correct by stating they are non-cellular and reproduce only inside host cells.
- Assuming all bacteria are pathogens; correct by noting many are harmless or beneficial.
- Placing malaria in the wrong group; correct by identifying the malarial pathogen as a protist.
- Assuming only humans are affected; correct by giving a plant disease example such as rose black spot.
- Confusing water-borne with airborne spread by stating droplets are water-borne; correct by classifying droplet transmission as airborne.
- Describing a route without linking it to a named disease; correct by pairing each route with an example.
- Saying viruses divide like bacteria; correct by stating viruses replicate inside host cells.
- Describing reproduction without linking it to harm; correct by adding how cell damage or toxins cause symptoms.
- Assuming reproduction happens outside the body; correct by stating it occurs inside the host.
- Thinking bacteria always cause harm by entering and destroying cells; correct this by stating that many bacteria cause illness through toxins acting outside cells.
- Confusing toxins with the bacteria themselves; correct this by describing the toxin as a separate poisonous chemical produced by the bacterium.
- Assuming all bacteria are pathogens; correct this by noting that many bacteria are harmless or beneficial, and only some produce harmful toxins.
- Saying viruses are a type of bacterium; correct this by stating viruses are not cells and are much smaller.
- Believing viruses can reproduce on their own; correct this by explaining they need a host cell's machinery.
- Thinking antibiotics can treat viral infections; correct this by stating antibiotics target bacteria, not viruses.