Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
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AIDS is a set of diseases caused by the destruction
of the immune system by infection with human
immunodeficiency virus (HIV). HIV is transmitted in
certain body fluids: blood, semen, vaginal secretions
and breast milk. HIV primarily infects economically
active members of populations in developing countries
and has an extremely adverse effect on social and
economic development.
The transmission of HIV can be controlled by using
barrier methods (e.g. condoms and femidoms) during
sexual intercourse. Educating people to practise safer
sex is the only control method currently available to
health authorities. Contact tracing is used to find people
who may have contracted HIV, so that they can be tested
and counselled.
Life expectancy can be greatly extended by using
combinations of drugs which interfere with the
replication of HIV. However, such treatment is expensive,
is difficult to maintain and has unpleasant side-effects.
There is no vaccine for HIV and no cure for AIDS.
TB is caused by the bacteria Mycobacterium tuberculosis
and M. bovis. M. tuberculosis is spread when people
infected with the active form of the disease release
bacteria in droplets of liquid when they cough or sneeze.
Transmission occurs when uninfected people inhale the
bacteria. This is most likely to happen where people live
in overcrowded conditions, and especially where many
sleep close together.
Many people have the inactive form of TB in their lungs,
but they do not have the disease and do not spread it.
The inactive bacteria may become active in people who
are malnourished or who become infected with HIV. M.
bovis causes TB in cattle, but can be passed to humans.
Drugs are used to treat people with the active form
of TB. The treatment may take nine months or more
as it is difficult to kill the bacteria. Contact tracing is
used to find people who may have caught the disease.
These people are tested for TB and treated if found to
be infected. The BCG vaccine provides some protection
against TB, but its effectiveness varies in different parts
of the world.
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Measles is an extremely contagious disease caused
by a virus that inhabits the cells of the nasal cavity
and trachea. Infected people sneeze or cough out
droplets which contain millions of virus particles. If
these are inhaled by a person with no immunity, it
is almost certain that they will be infected with the
disease. Symptoms include fever and a rash. There is
no specific medicine for measles – treatment is rest and
medicine to reduce the fever. Measles is controlled in
economically developed countries by vaccination and
there are very few outbreaks of the disease, but is a
major disease in developing countries.
Public health measures are taken to reduce the
transmission of all of these infectious diseases, but to be
effective they must be informed by a knowledge of the
life cycle of each pathogen.
Antibiotics are drugs that are used to treat infections
caused by pathogenic bacteria. They are compounds
that are made by microorganisms and modified
chemically to increase their effectiveness. Penicillin
prevents the production of new cell walls in bacteria
and so does not affect viruses or human cells, neither
of which have cell walls. Not all antibiotics are effective
against all bacteria.
Resistance to antibiotics can arise because some
bacteria may, by chance, contain a resistance gene. The
bacteria survive when exposed to the antibiotic and can
then reproduce to form a large population of bacteria all
containing this gene. This is called vertical transmission
of resistance. Resistance can also be spread between
bacteria by horizontal transmission. This is when
plasmids are transferred between bacteria during
conjugation.
The widespread and indiscriminate use of antibiotics
has led to the growth of resistant strains of bacteria.
This poses a serious challenge to the maintenance of
health services in the 21st century.