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B5.4 Viral Diseases

GCSE Combined Science · AQA · Chapter B5: Communicable Diseases

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: measles, HIV and tobacco mosaic virus: symptoms, how they spread and how the spread is controlled. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe the symptoms, spread and prevention of measles
  • Describe how HIV affects the immune system, how it spreads and how it is controlled with antiretroviral drugs
  • Explain how tobacco mosaic virus (TMV) reduces plant growth, and how its spread is reduced
  • Interpret data about viral diseases

Key words

  • Measles: A serious, highly infectious viral disease that causes fever and a red skin rash. It can be fatal.
  • HIV: Human immunodeficiency virus. It attacks the body's immune cells.
  • AIDS: Late-stage HIV infection, when the immune system is so damaged it cannot deal with other infections or cancers.
  • Antiretroviral drugs: Medicines that stop HIV reproducing, so the immune system is not destroyed.
  • TMV: Tobacco mosaic virus: a plant virus that causes a mosaic pattern of discolouration on leaves.

Measles

  • Symptoms; Measles Fever and a red skin rash. It can be fatal if complications arise. In adults it can cause more serious problems, such as harm to an unborn baby.
  • How it spreads; Measles By breathing in droplets from the sneezes and coughs of an infected person. It is very infectious.
  • Treatment; Measles No drug kills the virus. Patients rest and are isolated while the immune system fights it.
  • Prevention; Measles Vaccination. Young children in the UK are offered the MMR vaccine (measles, mumps and rubella).

Key fact: When the percentage of children vaccinated falls, outbreaks become more likely, because the virus can find people who are not immune.

HIV and AIDS

HIV first causes a short flu-like illness. Many people then have no symptoms for years. The virus attacks the body's immune cells. Unless it is controlled with antiretroviral drugs, the immune system becomes so badly damaged that it can no longer deal with other infections (such as tuberculosis) or cancers. This late stage is called AIDS.

  • Sexual contact; How spread is reduced Using condoms; testing and treatment
  • Exchange of body fluids, e.g. blood when drug users share needles; How spread is reduced Not sharing needles; needle exchange schemes; screening donated blood
  • From mother to baby in the uterus, at birth or in breast milk; How spread is reduced Antiretroviral drugs during pregnancy

Key fact: There is no cure or vaccine for HIV, but antiretroviral drugs keep the amount of virus so low that people can live a long, healthy life.

Tobacco mosaic virus (TMV)

TMV is a widespread plant pathogen that affects many species, including tobacco and tomatoes. It causes a distinctive mosaic pattern of discolouration on the leaves: pale or yellow patches where the virus has damaged the chloroplasts. Leaves may also be wrinkled.

Worked example: explaining the effect on growth

Why does a pepper plant with TMV produce fewer peppers?
Discoloured areas have less chlorophyll, so less light is absorbed for photosynthesis.
Less glucose is made, so less is available for growth and for making fruit.

Answer: Less photosynthesis, so stunted growth and a lower yield

TMV enters through damaged parts of a plant. It spreads by contact between leaves, and on the hands, tools and clothes of growers. There is no cure, so growers prevent it: they grow resistant varieties, remove infected plants straight away, remove nearby weeds that may carry the virus, and keep hands and tools clean.

Common mistakes

  • Saying HIV and AIDS are the same thing. AIDS is the late stage of HIV infection.
  • Saying people die "from AIDS itself". They die from other infections or cancers because their immune system is damaged.
  • Saying TMV kills plants directly. It reduces photosynthesis, so growth and yield fall.
  • Saying antibiotics treat measles or HIV. Antibiotics do not work on viruses.
  • Thinking HIV spreads by touching, sharing cups or coughing. It spreads through sexual contact and body fluids such as blood.

Revision questions

Try these, then check your answers below each one.

Question 1. Which are the main symptoms of measles? (Choose from: A fever and a red skin rash, Pain when urinating and a discharge, Vomiting and diarrhoea, A mosaic pattern on the skin.)

Answer: A fever and a red skin rash

Working: Measles causes a fever and a red skin rash. It can be fatal if complications arise.

Question 2. The diagram shows two tomato leaves. Leaf B has a mosaic pattern caused by tobacco mosaic virus (TMV). Why does a plant with leaves like B grow less well? (Choose from: The virus blocks the roots, Pale areas have less chlorophyll, so less photosynthesis takes place, The virus releases toxins into the fruit, The leaves lose too much water.)

Answer: Pale areas have less chlorophyll, so less photosynthesis takes place

Working: TMV damages chloroplasts. The pale patches have less chlorophyll, so less light is absorbed for photosynthesis. Less glucose is made, so growth is stunted.

Question 3. True or false? HIV can be passed on by shaking hands with an infected person.

Answer: False

Working: False. HIV is spread by sexual contact and by exchanging body fluids such as blood, for example when drug users share needles. It can also pass from mother to baby.

Related lessons

Continue in the full course

This lesson is section B5.4 (AGCSC0504) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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