Communicable Diseases » Viral Diseases

Viruses that cause disease

Viruses are pathogens. They live and reproduce inside cells, and this causes cell damage. In this lesson you will meet three viral diseases: two that affect people, measles and HIV, and one that affects plants, tobacco mosaic virus.

Key terms:

• Measles: a viral disease with symptoms of fever and a red skin rash.
• HIV: a virus that attacks the body's immune cells.
• AIDS: late stage HIV infection, when the immune system is so badly damaged it can no longer deal with other infections or cancers.
• Tobacco mosaic virus (TMV): a widespread plant pathogen that gives leaves a mosaic pattern of discolouration.

Measles

Measles is a viral disease. Its symptoms are a fever and a red skin rash.

Measles is a serious illness. It can even be fatal if complications arise. This is why most young children are vaccinated against measles. You will find out how vaccines work in the lesson called Vaccination.

When an infected person sneezes or coughs, tiny droplets containing the virus are released into the air. Someone nearby can then breathe them in. This is an airborne way of spreading a disease, so it can pass quickly between people who are close together, such as children in a classroom.

HIV and AIDS

HIV stands for human immunodeficiency virus. It is spread in two main ways:

• Sexual contact.
• Exchange of body fluids, such as blood. This happens when drug users share needles.

HIV is not spread by sneezes and coughs in the way measles is.

What HIV does inside the body

HIV infection happens in stages.

1. Early stage: HIV initially causes a flu-like illness.
2. Without control: unless the virus is successfully controlled with antiretroviral drugs, it attacks the body's immune cells.
3. Late stage (AIDS): the immune system becomes so badly damaged that it can no longer deal with other infections or cancers.

Notice that the serious illness in AIDS comes from other infections and cancers. The damaged immune system cannot deal with them.

Tobacco mosaic virus (TMV)

Not all viruses infect animals. Tobacco mosaic virus is a widespread plant pathogen. It affects many species of plants, including tomatoes.

TMV gives a distinctive mosaic pattern of discolouration on the leaves. A mosaic is a pattern made of patches, a bit like a picture made from small tiles. The leaf has patches that are a different colour from the normal green.

Remember that plants make their own food by photosynthesis in their leaves. If the leaves are damaged, less food is made, and the plant cannot grow as well.

Why do viral diseases matter?

Each of these three viruses causes real harm, in a different way.

• Measles is serious and can be fatal if complications arise. This is why most young children are vaccinated. It also spreads through the air in droplets, so it can reach many people.
• HIV damages the immune system, which is the body's defence against pathogens. A person with a badly damaged immune system cannot deal with other infections or cancers. That is why late stage HIV infection is so serious.
• TMV harms crops such as tomatoes. It is a widespread pathogen, so it affects the growth of many plants.

Spotting the symptoms matters too. A fever and a red skin rash point to measles. A flu-like illness is how HIV first shows itself. A mosaic pattern on the leaves points to TMV.

Worked example: link the symptom to the disease

Comparing the three diseases

Exam-style question

(a) Describe the symptoms of measles. [2 marks]

(b) Explain why measles spreads easily between children in a classroom. [2 marks]

(c) Tobacco mosaic virus affects tomato plants. Explain why the growth of an infected plant is affected. [2 marks]