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Microorganisms Viruses Information

Virus / December 12, 2016

Infectious diseases are caused by microorganisms such as viruses, bacteria, fungi or parasites and can spread between individuals.

What is an infectious disease?

  • Infectious diseases are caused by microorganisms such as viruses, bacteria, fungi or parasites.
  • Microorganisms that cause disease are collectively called pathogens.
  • Pathogens cause disease either by disrupting the bodies normal processes and/or stimulating the immune system to produce a defensive response, resulting in high fever, inflammation and other symptoms.
  • Infectious diseases can be spread from one person to another, for example through contact with bodily fluids, by aerosols (through coughing and sneezing), or via a vector, for example a mosquito.
  • Close up of a mosquito feeding on human blood.
    Image credit: Shutterstock

  • Infectious diseases are one of the leading causes of death worldwide.
  • Many diseases become difficult to control if the infectious agents evolve resistance to commonly used drugs:
  • For example, bacteria can accumulate mutations in their DNA or acquire new genes that allow them to survive contact with antibiotic drugs that would normally kill them.
  • What causes an infectious disease?


  • Viruses are tiny infectious agents that replicate only in the living cells of other organisms.
  • Viruses have a very simple structure consisting of genetic material in the form of DNA or RNA within a protein capsule.
  • They can infect all types of life forms, from animals to plants and bacteria to amoebae.
  • Viruses can be spread in many ways including:
  • From plant to plant by insects that feed on plant sap. For example, Potato virus Y which is spread by aphids.
  • From animal to animal by blood-sucking insects. For example, Dengue virus which is spread by mosquitos.
  • Spread by aerosols (through coughing and sneezing). For example, influenza virus.
  • Spread by not washing hands after going to the toilet. For example, norovirus or rotavirus.
  • Spread by sexual contact. For example, HIV and Human Papillomavirus (HPV).
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