5.6 Viral Diseases

A free GCSE Biology lesson for AQA (8461) 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
• Interpret data about viral diseases

Key words

• Measles: A serious 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: The late stage of 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 is a serious illness that can be fatal if complications arise, such as pneumonia or swelling of the brain.
• How it spreads; Measles By inhaling 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, measles 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. The virus then 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 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 during birth or breastfeeding; 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 and are very unlikely to pass it on.

Tobacco mosaic virus (TMV)

TMV is a widespread plant pathogen. It affects many species, including tomatoes and tobacco. It causes a distinctive mosaic pattern of discolouration on the leaves, where parts of the leaf lose their green colour.

Worked example: explaining the effect on growth

Why does a tomato plant with TMV grow less well?
Discoloured areas have less chlorophyll, so less light is absorbed for photosynthesis.
Less glucose is made, so less is available for respiration and for making new cells and fruit.

Answer: Less photosynthesis, so stunted growth and fewer tomatoes

TMV spreads by contact between leaves, and on the hands, tools and clothes of growers. There is no cure. Growers remove and destroy infected plants, keep tools and hands clean, control insect pests and grow TMV-resistant varieties.

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: Fever and a red skin rash, Discharge and pain on urinating, Vomiting and diarrhoea, Recurrent fevers every few days.)

Answer: 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. True or false? HIV can be passed on when drug users share needles.

Answer: True

Working: True. HIV spreads by sexual contact and by exchange of body fluids such as blood, for example when needles are shared.

Question 3. What does HIV attack in the body? (Choose from: Red blood cells, The body's immune cells, Nerve cells, Liver cells.)

Answer: The body's immune cells

Working: HIV attacks the body's immune cells. Without antiretroviral drugs, the immune system is badly damaged (AIDS).

Related lessons

• Previous lesson: 5.5 Preventing Infections
• Next lesson: 5.7 Bacterial Diseases
• In this chapter: 5.1 Health and Disease
• In this chapter: 5.2 Pathogens and Disease
• In this chapter: 5.3 Growing Bacteria in the Lab

Continue in the full course

This lesson is section 5.6 (AGCBH0506) of our free GCSE Biology 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 Biology course.