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Summary CIE A level Biology notes on unit 10 - Infectious Disease
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Everything you need to know about unit 10 of the CIE A level Biology course, including diagrams and notes.
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10.1 - Infectious Diseases:• An infectious disease is a disease resulting from the presence of pathogens including, viruses, bacteria, fungi and Protozoa - these
pathogens are transmissible (directly or indirectly) and are able to cause disease in animals or plants (specific or cross-species).
• Alternate diseases can be inherited of be non-infectious.
• Some diseases can be symptom-free in carrier species - Intermediate hosts can act as mixing vessels for the pathogen.
Cholera:
• Cholera is caused by the bacterium Vibrio cholera.
• It is most commonly a water-borne-faecal/oral disease in areas of poor sanitation (e.g. untreated sewage) and contaminated food - Can
also be transmitted by flies or other vectors.
◦The disease is transmitted indirectly, rather than by direct contact
between people.
◦75% of people are symptomatic, therefore they don’t know they are
spreading large quantities of the bacteria in their faeces.
• The bacteria moves from the stomach to the small intestine, and multiplies to
release a toxin called choleragen, which disrupts the epithelium and causes
diarrhoea - loss of minerals and salts
Treatment:
• Cholera can be treated with solutions of salts and glucose, to rehydrate the
body after losing salts in faeces.
• If they can drink, then they’re given oral rehydration therapy, as the glucose can
be absorbed into the blood and take ions into it.
• Vaccine only has short term protection so it is no longer recommended - hard
to make because there’s so many strains.
Factors affecting prevention and control:
• Developing countries which have had a major increase in population, but lack the economic strength and development of facilities such
as sewage treatment etc, lack the money to prevent the disease and can build up significant debt - puts large burdens on countries as
they have to borrow money from abroad.
• Raw sewage used to fertilise crops, inadequate cooking and washing in dirty water in LIDC countries can cause disease spread.
• There are often outbreaks of cholera after natural disasters such as earthquakes, because the water and sewage systems are
destroyed, as well as the lack of access to clean water.
• Lack of education on the matter in poor countries means that they lack the understanding of basic measures such as boiling water.
Malaria:
• Malaria is caused by the protoctist of plasmodium (theres 4 types - most common is Plasmodium falciparum).
• Female anopheles mosquitos feed on human blood containing plasmodium, and then the male and female gametes of parasites fuse in
the mosquitos stomach, which divide and multiply to produce thousands of parasites.
• When the anticoagulant is injected into another human, the pathogen passes through the mosquitos salivary glands and enters the
humans blood stream, which pass into the liver.
• The parasites leave the liver and re-enter the blood cells to multiply
• Symptoms of high temp, headaches, vomiting etc.
Treatment:
• Treatment by prophylactics such as ACTs (preventative drugs) which affect the parasite lifecycle - many parasites are now resistant, so
they are now prescribed two or more drugs that work against the parasite in different ways.
• There’s still struggle to produce the vaccine, but if treated correctly, prevention is better by reducing mosquitoes and biting (e.g. nets,
spray/tablets and antibiotics).
• Humans can be naturally protected by it if they possess the sickle cell trait in their genes, as the cells are a hostile environment for the
parasite.
Factors affecting prevention and control:
• Malaria is found mainly in warm and humid areas because the mosquitos prefer these conditions - they struggle at high altitudes and far
from fresh water sources.
• Highly populated cities, as well as poorly designed areas are susceptible due to easy spread and access to one another.
• They can breed in stagnant pools such as in agriculture, so many poorer areas are susceptible.
• Can be spread by blood transfusion, dirty needles and across the placenta due to mixing of blood.
• Immunity can be possible by repeated infection by different strains, but only short term - more dangerous in non-endemic areas.
HIV/AIDS:
• AIDS is caused by the Human immunodeficiency virus.
• Viral RNA is converted to DNA in the host, which infects and destroys helper T cells, leading to opportunistic diseases (degraded
immune system).
• It’s spread through bodily fluids, most commonly needles, blood transfusions or sexual contact (as well as placenta and breast milk).
• People will HIV can be symptom free for many years afterwards, due to the long latent stage, so have no knowledge of passing on the
disease.
• AIDS develops after HIV and it leads to many infections such as pneumonia, dementia etc.
Treatment:
• There’s no cure or vaccine for HIV/AIDS.
• There’s been a slow development of people surviving with drugs - anti-retrovirals - but these are expensive and have many side effects
such as headaches, nerve damage etc.
• Can be controlled by increased public health awareness - the use of condoms, using clean needles, contact tracing
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