Chapter 12- Pathogens and Defence Against Disease

12.1 Pathogens and disease

Pathogens (fungi, bacteria, viruses and protocistans) are microbes that cause disease
Viruses are very small. They inject their DNA into body cells, which use genes in the viral DNA to produce new viruses, which enter more body cells.
Common colds, flu, measles, mumps, rubella and chicken pox are caused by viruses. There are only a few antiviral drugs, but scientists have produced many vaccines for viral diseases
Bacteria are bigger. They multiply rapidly and cause disease by direct damage to body tissue and producing toxins which enter the blood stream.
MRSA, TB, cholera and pneumonia are caused by bacteria.Antibiotics kill bacteria.
Some small fungi live on the skin (warm and damp. Athlete's foot, thrush and ringworm are fungal infections.
Protocistans are a group of single-celled organisms, some cause disease
Malaria kills millions of people and is caused by plasmodium. The mosquito acts as a vector for the disease; the plasmodium is transferred into a person's blood when a mosquito bites them. It then infects the liver and the red blood cell
Amoebic dysentery is an intestinal infection, causing severe diarrhoea. The amoeba enters the gut through contaminated food

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There are antifungal creams and drugs, but avoidance is best. Fungi are easily picked up in warm, damp places (eg. changing rooms) and malaria is prevented with drugs (eg quinine) and avoiding mosquito bites
Close contact (eg touching infected skin) is the most common way to pick up disease
Droplet infection is when coughing and sneezing spreads droplets of water into the air, and are breathed in
Contaminated food and drink spread disease
Houseflies can act as vectors by carrying bacteria on their bodies

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The skin is a physical barrier against pathogens.
If bacteria are inhaled, they are trapped in mucus and are wafted up by cilia to be exhaled of swallowed
Stomach acid (also acidic secretions from the vagina) kills pathogens, as it denatures the enzymes.
When you get cut, the blood clots and a scab forms; this prevents entry of pathogens
By being hygienic and clean we can prevent infection.
If a large number of pathogens enter the body, it responds by detecting microbes, and producing antibodies and anti-toxins to combat them
As viruses are found inside body cells, it is hard to treat them without damaging cells also. Vaccines prevent the viruses entering the cells
Alexander Fleming was the first person to discover that bacteria are killed by penicillin

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Antibiotics work by damaging the walls of bacteria or by interfering with its growth mechanism
Some bacteria have developed mutations that make them resistant to antibiotics. If there are only a few mutant bacteria, the body's immune system can destroy them whilst the antibiotics work on normal bacteria
But if there are large numbers in the body, the person becomes ill and the antibiotics useless
All organisms compete for food and space, so large colonies of bacteria are in competition. If the normal bacteria are killed off by antibiotics, then the few remaining resistant bacteria are able to multiply rapidly without competition. This is natural selection
Overuse of antibiotics has caused the MRSA "superbug" to develop. People who are in poor health in hospital are vulnerable to MRSA. More hand wash and better basic hygiene on wards have been introduced in hospitals to combat MRSA
The large intestine is full of safe bacteria and a few C.diff. bad bacteria. If antibiotics are prescribed, the safe bacteria are killed and the C.diff. prosper

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