Causes of disease - Chapter 1

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Unit 1 - Causes of disease

Pathogens  - 1.1


Microorganism = General term used to describe a single celled organism that is too small to be seen without a microscope.


Most microorganisms do not cause disease and live happily in our bodies however, there are some, which do cause disease, and these are called pathogens.


Disease = A description of certain symptoms, either physical, mental or both. Disease also suggests a malfunction of body or mind, which has an adverse affect on good health. It has mental, physical and social aspects.


For a microorganism to be considered a pathogen it must:

·      Gain entry to the host

·      Colonize the tissues of the host

·      Resist the defences of the host

·      Cause damage to host tissues


Pathogens include bacteria, viruses and fungi.


If a pathogen gains entry into the host and colonises the tissue it is referred to as an infection. When an infection occurs and has recognisable symptoms, it is referred to as a disease. When a pathogen is transferred to another person it is known as transmission.


How do microorganisms get into the body?

·      By penetrating one of the organisms interfaces with the environment

o   Boundary linking two systems

·      The skin is an effective barrier to infection

o   Invasion normally only occurs when there is an abrasion on the skin surface

·      Some parts of the body are moist and have a large surface area, this allows for easy entry of molecules, including pathogenic molecules.

Common points of entry include:

·      The gas-exchange system

o   Gas exchange surfaces

o   Influenza, TB and bronchitis infect in this way

·      The digestive system

o   Food and water can carry pathogens into the stomach and intestines via the mouth

§  Cholera, typhoid and dysentery enter the body this way


In order to prevent the entry of pathogens:

·      A mucus layer covers the exchange surfaces

o   Makes them harder to penetrate

·      Production of enzymes that kill the pathogen

·      Production of stomach acid, which kills pathogens


How do pathogens cause disease?

Two ways:

1.     Damaging host tissues - the number of pathogens causes damage as it prevents tissues from functioning properly. Viruses inhibit the synthesis of DNA, RNA and proteins in the host cells. Many pathogens break down the membranes of the host cells.

2.     Producing toxins - most pathogens produce toxins. The Cholera bacterium produces a toxin that leads to excessive water loss from the lining of the intestines.


Some diseases have a single cause (malaria), whereas others have multiple causes (heart disease). Pathogens, lifestyle and genetic factors can all cause disease.


How fast a pathogen causes disease corresponds with how rapidly the pathogen divides. Some pathogens only cause damage when there is a large number present whereas, other pathogens can cause damage


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