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`Microorganism' is a general term for a single-celled organism that is too small to
be seen without a microscope. Microorganisms include bacteria and viruses.
Many microorganisms live more or less permanently in our bodies benefiting
from so doing, but causing no harm. Some of these microorganisms are
beneficial to us. Other microorganisms however, cause disease and these are
called pathogens.…read more

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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 host tissues…read more

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Pathogens include bacteria, viruses and fungi
If a pathogen gets into the host and colonises its tissue an infection results.
Disease occurs when an infection leads to recognisable symptoms in the host.
When a pathogen is transferred from on individual to another it is known as
transmission.…read more

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Pathogens normally get into the body by penetrating one of the organisms
interfaces with the environment. An interface is a surface or boundary linking
two systems, in this case linking the external environment with the internal
environment with the body.
One of these interfaces is the skin. However, as skin forms a thick, continuous
layer, it is an effective barrier to infection. Invasion normally occurs when the
skin is broken, ie cuts or animal/insect bites.
Some of the interfaces have evolved to allow exchange between internal and
external environments. As a result the body linings at these points are thin,
moist, have a large surface area and are well supplied with blood vessels. Just
as these features make for easy entry of molecules, so they also make for easy
entry of pathogenic microorganisms. Interfaces of the body are hence
common points of entry.…read more

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Gas · Many pathogens enter the body through the gas
exchange exchange system. Pathogens which cause influenza,
tuberculosis and bronchitis infect in this way.
Digestive · Food and water may carry pathogens into the stomach
and intestines via the mouth. Cholera, typhoid and
system dysentery pathogens enter the body by this means.…read more

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