Viral diseases — AQA GCSE Combined Science
Test yourself on Viral diseases with AQA GCSE practice questions.
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Viral diseases explained
Measles is caused by a virus and is highly contagious.
Read the full explanation
After a person inhales droplets released by an infected person's sneeze or cough, the virus multiplies in the airways and spreads through the body. Early symptoms include fever and a red skin rash; the rash often appears a few days after the fever begins. Most people recover, but complications such as pneumonia or encephalitis can occur, and measles can be fatal. Because measles spreads easily through the air, vaccination of young children protects individuals and reduces transmission in the population. The MMR vaccine contains weakened measles virus and stimulates immunity without causing the disease.
HIV initially causes a flu-like illness. Unless successfully controlled with antiretroviral drugs the virus attacks the body’s immune cells. Late stage HIV infection, or AIDS, occurs when the body's immune system becomes so badly damaged it can no longer deal with other infections or cancers. HIV is spread by sexual contact or exchange of body fluids such as blood which occurs when drug users share needles.
HIV is a virus that first causes a flu-like illness, then, if not controlled by antiretroviral drugs, it attacks the body's immune cells. This progressive damage means the immune system eventually cannot deal with other infections or cancers; this late stage is AIDS. HIV spreads by sexual contact or by exchange of body fluids, for example blood when drug users share needles. A useful model is a slow siege: the virus does not directly cause AIDS, but destroys the defenders so other pathogens or cancers can invade. Antiretroviral drugs suppress viral replication, so a person can live with HIV without progressing to AIDS.
Tobacco mosaic virus (TMV) is a widespread plant pathogen affecting many species of plants including tomatoes. It gives a distinctive ‘mosaic’ pattern of discolouration on the leaves which affects the growth of the plant due to lack of photosynthesis.
Tobacco mosaic virus (TMV) is a widespread plant pathogen that infects many species, including tomatoes. It causes a distinctive mosaic pattern of discolouration on leaves: patchy light and dark areas rather than uniform green. The discoloured areas contain less chlorophyll, so the leaf absorbs less light for photosynthesis. Less photosynthesis means less glucose is made for growth, so the plant grows poorly. TMV spreads between plants by direct contact, including contaminated hands, tools and clothing, and via infected sap; it is not spread by insect vectors such as aphids. There is no cure, so control relies on removing infected plants, disinfecting tools and washing hands to reduce spread. A useful comparison is a healthy leaf, which is uniformly green, with an infected leaf showing patchy discolouration.
Your focus
- Identify the symptoms and transmission route of measles.
- Explain why measles can be serious and why vaccination is used.
- Describe how vaccination of young children reduces the spread of measles.
Show all 10 objectives
- Describe the initial flu-like illness caused by HIV and the later damage to immune cells.
- Explain how antiretroviral drugs control HIV and why late stage HIV infection is called AIDS.
- Identify sexual contact and exchange of body fluids such as blood through shared needles as routes of HIV transmission.
- Describe the mosaic pattern of discolouration caused by TMV on leaves.
- Explain how TMV reduces photosynthesis and therefore affects plant growth.
- Identify that TMV is a widespread pathogen affecting many plant species, including tomatoes.
- State that TMV spreads by contact, not by insect vectors such as aphids.
Viral diseases exam tips
Marking Points
- Measles is caused by a virus, not by bacteria or fungi.
- Symptoms include fever and a red skin rash.
- Transmission occurs by inhalation of droplets from sneezes and coughs.
- Measles can be serious and may be fatal if complications such as pneumonia or encephalitis develop.
- Most young children are vaccinated against measles to prevent infection and reduce spread.
- Vaccination protects the individual and can reduce transmission within the population.
- HIV initially causes a flu-like illness, which may be mild and non-specific.
- Without successful control by antiretroviral drugs, HIV attacks the body's immune cells.
- Late stage HIV infection, or AIDS, occurs when the immune system is so badly damaged that it can no longer deal with other infections or cancers.
- HIV is spread by sexual contact.
- HIV is also spread by exchange of body fluids such as blood, which occurs when drug users share needles.
- Antiretroviral drugs control HIV but do not cure it; they reduce the risk of progression to AIDS.
- TMV is a widespread plant pathogen affecting many species of plants, including tomatoes.
- It gives a distinctive mosaic pattern of discolouration on the leaves, with patchy light and dark areas rather than uniform green.
- The discoloured areas contain less chlorophyll, so less light is absorbed for photosynthesis.
- Reduced photosynthesis lowers glucose production, which affects the growth of the plant.
- TMV spreads by contact between plants, including contaminated tools, hands and sap; it is not spread by aphids or other insect vectors.
- Control relies on removing infected plants and reducing contact spread, for example by disinfecting tools, because there is no cure.
Examiner Tips
- 💡Name both symptoms and the transmission route when describing how measles spreads.
- 💡Use the word 'droplets' and link them to sneezes or coughs to show the method of inhalation.
- 💡When explaining vaccination, refer to protecting young children and reducing the chance of an outbreak.
- 💡Use the phrase 'immune cells' rather than 'white blood cells' alone, and link their destruction to the inability to deal with other infections or cancers.
- 💡When asked about transmission, give both named routes: sexual contact and exchange of body fluids such as blood through shared needles.
- 💡If a question asks why antiretroviral drugs matter, explain that they control the virus and prevent progression to AIDS, not that they cure HIV.
- 💡Link the mosaic pattern to chlorophyll loss, then to reduced photosynthesis and reduced growth in a clear chain.
- 💡Name at least one other plant affected, such as tomatoes, to show the pathogen is widespread.
- 💡When describing control, mention removing infected plants and reducing contact spread, such as disinfecting tools, rather than suggesting a cure or blaming aphids.
Common Mistakes
- Describing measles as a bacterial disease: correct this by naming the measles virus as the pathogen.
- Saying the rash is the only symptom: correct this by including fever and noting that complications can occur.
- Confusing droplet inhalation with direct skin contact: correct this by stating the virus is breathed in from airborne droplets.
- Saying HIV and AIDS are the same thing. Correction: HIV is the virus; AIDS is the late stage of infection when the immune system is badly damaged.
- Claiming HIV directly kills the person by attacking the lungs or other organs. Correction: HIV attacks immune cells, so other infections or cancers become life-threatening.
- Stating that HIV spreads by casual contact such as hugging or sharing food. Correction: it spreads by sexual contact or exchange of body fluids such as blood, for example through shared needles.
- Saying TMV kills the plant directly. Correction: it reduces growth because photosynthesis is reduced, and the plant may become weakened.
- Confusing the mosaic pattern with a mineral deficiency. Correction: TMV is caused by a virus and shows a patchy mosaic of discolouration, not a uniform yellowing.
- Stating that TMV spreads by aphids or other insect vectors. Correction: TMV spreads by contact, including contaminated tools, hands and sap; aphids are not vectors for TMV.
- Stating that TMV affects only tobacco. Correction: it is a widespread pathogen affecting many species, including tomatoes.