Causes of disease - chapter 1

Notes for the whole of AQA biology 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
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
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

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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.…read more

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Physical activity
Risk factors and coronary heart disease
CHD is the largest cause of death in the UK.
Factors we can control:
Smoking - between 2 and 6 times more likely to develop CHD.…read more


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