3.2 Viral diseases: measles, HIV and TMV

Viruses and disease

Viruses reproduce inside cells and damage them. AQA asks about two human viral diseases and one plant viral disease.

Measles

• Spread: breathing in droplets from the coughs and sneezes of an infected person.
• Symptoms: a fever and a red skin rash.
• Measles is a serious disease that can be fatal if there are complications, such as damage to the brain or lungs.
• Control: most young children are vaccinated against measles. There is no treatment, so infected people are isolated.

HIV

• Spread: by sexual contact or the exchange of body fluids, such as blood when drug users share needles, or from mother to baby.
• Symptoms: at first, a flu-like illness. The virus then attacks the body's immune cells (white blood cells).
• If it is not treated, the immune system becomes so badly damaged that the body can no longer deal with other infections or cancers. This late stage is called AIDS.
• Control: antiretroviral drugs stop the virus reproducing, so people with HIV can live long, healthy lives. Using condoms and not sharing needles reduce spread.

Tobacco mosaic virus (TMV)

• TMV infects many species of plant, including tomatoes and tobacco.
• Symptoms: a distinctive mosaic pattern of yellow and light green discoloured patches on the leaves.
• The discoloured parts have less chlorophyll, so the plant carries out less photosynthesis, which reduces its growth.
• Spread: by contact between plants, on the hands or tools of people, and by insects.
• Control: there is no treatment, so infected plants are removed and destroyed, and growers use TMV-resistant varieties.