Causes of Disease, Biology unit 1.1

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For a microorganism to be considered a pathogen it must:

  • Gain entry to the host
  • Colonise the tissues of the host
  • Resist the defences of the host
  • Cause damage to the host tissues

Certain systems of the body are specialized in exchange of material between internal and external environments. These exchange surfaces are thin (short diffusion pathway), moist, have a large surface area, and are well supplied by blood vessels. This makes it easier for pathogens to pass into the body too.

Microorganisms get into the body through:

  • Cuts, insect bites and abrasions of the skin
  • The gas-exchange system- eg influenza, tuberculosis and bronchitis
  • The digestive system- eg cholera, typhoid and dystentery

To prevent entry, the body's natural defenses include:

  • A mucous layer that covers exchange surfaces and forms a thick sticky barrier that is difficult to penetrate
  • Production of enzymes that break down the pathogens
  • Production of stomach acid, to kill microorganisms

Pathogens affect the body by damaging host tissues. Viruses inhibit the synthesis of DNA, RNA and proteins by the host cells, often damaging these cells. The pathogens also produce toxins that cause a damaging effect, such as the cholera toxin which makes water leak from the cells…


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