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Biology Unit 4
Topic 6: Infection, Immunity and Forensics
11. Explain how bacterial and viral infectious diseases
have a sequence of symptoms that may result in
death, including the diseases caused by
Mycobacterium tuberculosis (TB) and Human
Immunodeficiency Virus (HIV).
Both viruses and bacteria may enter the bodies of living things and, due to their own life processes,
cause symptoms which can, in extreme cases, lead to death of the host organisms.
TB is most commonly caused by the bacterium Mycobacterium tuberculosis which is spread by
droplet infection. Crowded living or working conditions increase the likelihood of it spreading.
People who are malnourished, ill or have problems with their immune system are more vulnerable to
the disease and are much more likely to develop active TB than healthy, well-fed individuals.
The other common source of infection is from the bacterium Mycobacterium bovis, which particularly
affects cattle. People become infected from drinking infected milk or living and working in close
contact with cattle.
In TB the first infection may have no symptoms but tubercles form in the lungs due to the
inflammatory response of the person's immune system. Some bacteria may survive inside
the tubercles, due to their thick waxy coat. They lie dormant, but if the immune system is not
working properly they can become active again.
In active TB, lung tissue is slowly destroyed by the bacteria, causing breathing problems. The
patient develops a serious cough, loses weight and appetite and may suffer from fever.
Mycobacteria also target T cells, reducing the production of antibodies and so disarming a
critical part of the immune system. The bacteria are temperature sensitive and stop
reproducing above 42°C.
Eventually TB causes death, either because the individual cannot get enough oxygen from
the air through their damaged lungs, or because their organs fail through lack of nutrition.
And, since TB affects their immune system itself, sufferers often become very vulnerable to
opportunistic infections such as pneumonia which may be what finally kills them.
Text Book: p. 116 123
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Biology Unit 4
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