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

Viruses: Basics

• Many common diseases are caused by viruses, for example:

  • the common cold

  • influenza, or 'flu

• Viruses reproduce rapidly by inserting their genetic material into host cells and using the host cells 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 serious viral illness that can be fatal in complicated cases

Transmission

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

• This makes it very infectious

Symptoms

• The main symptoms of measles are:

  • fever

  • red skin rash

• Most people recover well from measles, but it can cause lasting issues such as blindness and brain damage

Control

• Most young children are vaccinated against measles due to its potentially serious effects

  • Vaccination programmes have been highly effective, but the infectious nature of measles means that it is still present in populations

• If someone becomes infected with measles, they need to be isolated to stop the spread of the virus

HIV

• HIV (Human immunodeficiency virus) is a virus that can eventually lead to Acquired immunodeficiency syndrome (AIDS)

Transmission

• The virus is spread by:

  • sexual contact

  • the exchange of blood; for example, it can be transmitted when drug users share needles

  • from mother to child across the placenta or during birth

Symptoms

• HIV starts with a flu-like illness

• It can then stay hidden in the immune system for many years

• If untreated it can attack cells of the immune system until it can no longer deal with other infections or cancers

  • This is late-stage HIV infection, known as AIDS

Control

• Antiretroviral drugs taken early in the infection can now effectively control the disease

Tobacco Mosaic Virus

• Tobacco mosaic virus (TMV) is a widespread plant pathogen that infects many plant species, e.g. tomato plants and cucumbers

Transmission

• The virus is spread by:

  • direct contact with an infected plant

  • growing plants in infected soil

Symptoms

• Infected plants develop a distinctive mosaic pattern of discolouration on their leaves as the virus infects the chloroplasts

• Plant growth will be reduced due to a reduction in photosynthesis; this reduces crop yield

Control

• 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