Viral Diseases (AQA GCSE Combined Science: Trilogy: Biology): Revision Note

Viruses: Basics

• There are many common diseases caused by viruses; for example, the common cold and the flu are caused by viruses

• Viruses do not fulfil the seven life processes, so they are not usually classified as living organisms

  • Viruses do not have nuclei, organelles, or cytoplasm like cells do, and so they have no way to monitor or create change in their internal environment

• Viruses reproduce rapidly by inserting their genetic material into host cells and creating new protein capsules to build new viral particles

• Once many copies have been made, the host cell may burst open releasing the viral particles which can go on to infect other cells

Measles

• Measles is a highly contagious and a potentially serious viral infectious disease that can be fatal if complications arise

• Measles is most often seen in children – most young children are vaccinated against it

Symptoms

• The main symptoms of measles are:

  • Fever

  • Red skin rash

• Most people recover well from measles; however, measles is a serious disease that can cause blindness and brain damage

How is it spread?

• The virus is spread by the inhalation of droplets from coughs and sneezes.

• This makes it very infectious

Treatment & prevention

• There is no treatment for measles, so if someone becomes infected they need to be isolated to stop the spread of the virus

• Measles is now greatly reduced in the UK as a result of improved living conditions and a vaccination programme for young children

HIV

• HIV (Human Immunodeficiency Virus) is a virus that can eventually lead to Acquired Immunodeficiency Syndrome (AIDS)

Symptoms

• HIV starts with a flu-like illness.

• If untreated it can travel to the lymph nodes and attack cells of the immune system

• It can stay hidden in the immune system for many years until the immune system is so badly damaged it can no longer deal with other infections or cancers (this is late-stage HIV infection, known as AIDS)

How is it spread?

• The virus is spread by:

  • Direct sexual contact

  • The exchange of bodily fluids such as blood; for example, it can be transmitted when drug users share needles

  • From mother to child during birth or in breast milk

Treatment & prevention

• There is no cure for HIV, although the use of antiretroviral drugs used early in the infection can now effectively control the disease to slow or halt the progress to AIDS

Tobacco Mosaic Virus

• Tobacco mosaic virus (TMV) was the first virus to be isolated by scientists

• It is a widespread plant pathogen that infects about 150 species of plants including tomato plants and cucumbers

Symptoms

• The symptoms of tobacco mosaic virus are the distinctive mosaic pattern of discolouration of the leaves as the virus infects the chloroplasts

• The plant will not grow as much due to the lack of photosynthesis – this reduces the yield of crops

How is it spread?

• The virus is spread by:

  • Plants in direct contact with an infected plant

  • The virus can stay in the soil for about 50 years

Treatment & prevention

• There is no treatment for tobacco mosaic virus and the best method of control is good field hygiene to prevent the spread of the virus

• Farmers are also using tobacco mosaic virus-resistant strains of crop plants