Viral Diseases

Viruses are not classified as living organisms because they do not complete the seven life processes:

  • Movement
  • Reproduction
  • Sensitivity
  • Respiration
  • Nutrition
  • Excretion
  • Growth

Viruses are made up of a short length of genetic material, which is surrounded by a protein coat. When describing them, we say “strains” instead of saying “species”.

  • Unlike bacteria, viruses cannot be killed by antibiotics


HIV (Human immunodeficiency virus) is a virus that damages cells in the immune system.

SymptomsHow it spreadsTreatment and prevention
• Starts off with a flu-like illnessIf left untreated, the virus attacks the patient’s cells

• Over time, the patient’s immune system becomes so badly damaged that it cannot fight off infections or cancers

This is a late-stage HIV infection, otherwise known as AIDS (Acquired Immunodeficiency Syndrome)
• Direct sexual contact

• The exchange of bodily fluids, for example, when drug users share infected needles

Transmission from mother to child during pregnancy, labour, delivery or breastfeeding
• There is no cure for HIV, however, antiretroviral drugs stop the virus from multiplying inside the patient

This prevents the virus from weakening the patient’s immune system, so they don’t go on to develop AIDS. The patient has to take these drugs for the rest of their life.

• Using condoms

• HIV-positive mothers using bottle feeding instead of breastfeeding


Measles is a highly contagious disease that is quite common in young children.

SymptomsHow it spreadsPrevention
• Fever

• Red skin rash

• Can lead to respiratory problems, brain damage and blindness – In severe cases, measles can be fatal
Spreads in droplets when the infected person coughs and sneezes – The virus passes to another person when they inhale the dropletsYoung children are vaccinated against measles to reduce transmission

Tobacco Mosaic Virus

Tobacco mosaic virus (TMV) is a plant pathogen that affects many species of plants, including tobacco, tomatoes and spinach.

SymptomsHow it spreadsTreatment and prevention
• It infects the chloroplasts of the plant’s leaves – This causes discolouration of the leaves from green to yellow, which forms a mosaic pattern

• The affected part cannot photosynthesize, which reduces the crop yield
Contact between diseased plants and healthy plants

Humans can act as vectors for TMV, as we cannot be infected

For example, cigarettes can contain TMV from an infected tobacco plant, so smokers can carry TMV through their hands and clothing
There is no cure for TMV, but farmers can carry out the following methods of control:

• Good field hygiene – Prevents the spread of the virus

• Growing TMV-resistant strains

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