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BIOL1 > Causes of Disease
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What is disease?

  • A disease is defined as a physical or mental disorder or malfunction with a characteristic set of signs or symptoms.
  • Diseases may be caused by a single factor such as a pathogenic microorganism or be multifactorial and have many causes some of which may depend on lifestyle.


  • Pathogenic microorganisms include bacteria, viruses and fungi.
  • Each pathogen has a specific method by which it causes disease.  Some such as the influenza virus damage host cells whereas others, such as Vibrio cholera produce toxins which have a harmful effect on the body.
  • The ability of the pathogen to cause disease depends on
    • Location - what tissue is colonised
    • Infectivity - how easily a bacterium can enter the host cell
    • Invasiveness - how easily a bacterium or its toxin spreads within the body
    • Pathogenicity - how a bacterium causes disease
  • Common entry points for pathogens are the gas exchange and digestive system.


  • A healthy lifestyle involves:
    • Maintaining a healthy weight
    • Taking regular exercise
    • Eating a balanced diet
    • Drinking a limited amount of alcohol
    • Not smoking
  • Lifestyle has an effect on human health:
    • Exposure to carcinogenic chemicals (i.e. those found in tobacco smoke) or radiation (i.e. X-rays) may increase a person's chance of contracting cancer
    • Coronary heart disease is associated with diets that are rich in fat and a sedentary lifestyle
    • Alterations to a person’s lifestyle can have a dramatic effect on the likelihood of them contracting these conditions