Communicable Diseases - one person can infect another
Examples explained, transmission, treatments, reduction, prevention
Doc Brown's biology exam revision study notes
What is a communicable disease? What
is a vector?
How can we minimise the spread of
communicable diseases?
How do you prevent the spread of
communicable diseases?
Describe and explain examples of
communicable diseases?
Sub-index for this page
(1)
Introduction to communicable diseases
(2)
Which types
of pathogens cause communicable
diseases?
(3)
How are pathogens spread?
(4)
Bacterial pathogen communicable diseases
- salmonella, E. coli
(5)
Bacterial pathogen communicable diseases
- cholera, tuberculosis (TB), stomach ulcers
(6)
Bacterial pathogen communicable diseases
- gonorrhoea, chlamydia (STDs)
(7)
Viral pathogen communicable diseases
- measles, chicken pox
(8)
Viral pathogen communicable diseases
- HIV (aids), HPV (both sexually transmitted diseases)
(9)
Viral pathogen communicable diseases
- ebola, hepatitis
(10)
Pathogens that attack plants
(11)
Examples of
protist pathogen communicable disease - malaria
(12)
More on how can we prevent, or reduce
the spread of communicable diseases
See also
Viruses - structure, reproduction and cell destruction
non-communicable diseases
Keeping healthy - our defences against
pathogens, fighting infectious diseases, vaccination
For evaluating data,
statistics, graphs and correlation see last section on
Keeping healthy -
non-communicable diseases - risk factors
Learning objectives for the pages on diseases and the body's defences
- Know that infectious diseases are caused by pathogens.
- Be able to
describe
how pathogens are spread, including:
- a) in water, including cholera bacterium
- You can be infected with a pathogen by
coming into contact with contaminated water - which is why swimming bath
waters are treated to kill bacteria with chlorine or ozone. In poor third
world countries the bacterial infection cholera, which causes diarrhoea and
dehydration, is readily spread in water contaminated with the faeces of
cholera sufferers. It is potentially very serious, particularly for the very
young and the very old and undernourished adults and children in poor third
world countries with poor sanitation.
- b)
by food, including Salmonella bacterium infection
- If you eat food contaminated with pathogens
the resulting food poisoning effects can be very unpleasant and potentially
very serious, particularly for the very young and the very old and the poor
of the third world. If food is kept too long at the wrong temperature, left
out in the open, or food like meat undercooked, you may be poisoned by the
bacterium salmonella.
- c) airborne (eg coughing, sneezing), including
influenza virus (causes flue)
- If you are suffering from a cough, chest
infection or flue etc. and you don't take precautions with a large
handkerchief or tissue, when you cough or sneeze you blast out into the air
a fine mist of water droplets containing millions of bacteria or viruses.
People around you breathe in you exhaled pathogens and potentially become
infected. Lots of people in a crowded room are great breeding places for
pathogens!
- d) by contact, including athlete’s foot fungus
infection
- You can be infected with a pathogen just by
touching a contaminated surface with e.g. your hand or foot. A common
example is the spread of athlete's foot, a fungal infection easily spread in
swimming bath surfaces, shower floors, towels i.e. anything an athlete's
foot carrier has been in contact with.
- e) by body fluids,
including HIV infection
- The HIV virus causes AIDS, a disease that
stops our immune system from functioning properly - you become more
susceptible to infectious diseases than a normal healthy person and the
condition is often fatal in the end, despite the best efforts of anti-viral
drugs. These kinds of pathogens can only be passed on by direct contact with
body fluids from another person e.g. from a HIV carrier's sperm during
sexual intercourse, or some body penetrating situation e.g. using the same
drug needle as a HIV carrier.
- f) by animal vectors (animals that spread
diseases), including:
- (i) housefly: dysentery bacterium
- The common housefly is a carrier of a nasty
protozoan bacterium. This pathogen causes dysentery, a disease that
expresses itself with severe diarrhoea and dehydration. Again this can have
serious consequences for the very young, the very old and the poor of
the third world.
- (ii)
Anopheles mosquito: malarial protozoan
- The mosquito is a carrier of protozoan
pathogen that causes the disease called malaria, a disease that causes
potentially fatal kidney and brain damage. This serious infectious disease
is passed onto another animal which is bitten by a mosquito - a mosquito
bite is a bit more serious than a bee or wasp sting!
- Be able to explain how the human body can be effective against attack from
pathogens, including:
- The body has different physical and chemical ways of protecting itself
against pathogens.
- a) Physical barriers – skin, cilia, mucus
-
Physical protection from
pathogens
-
Your skin and hairs and mucous
in the respiratory tract can stop a lot of the pathogen cells from entering
your body. The whole of the respiratory tract from the nasal passage, down
the trachea and into the lungs is covered with mucous and lined cilia (fine
hairs that can move freely at their ends). The mucous traps dust and
bacteria before they can get down into the lungs and the cilia move the
mucous along from the lungs up to the nasal passage -and then you can blow
your nose!
-
Skin in good condition acts as a
very effective barrier against pathogens. When a cut in the skin occurs,
small sections of cells called platelets help the blood to clot quickly to
seal the wound (seal = scab when dry) and prevent microorganisms entering
the skin tissue or blood stream. The greater the concentration of platelets
in the blood the faster the clotting process ('sealing') can occur.
- b) Chemical defence – hydrochloric acid in the stomach, lysozymes in tears
-
Chemical protection by killing
pathogens
-
In tears our eyes produce
chemicals called lysozymes that kill bacterial microorganisms on the surface
of the eye.
-
Your stomach contains quite
concentrated hydrochloric acid which kills the majority of pathogenic
bacteria - sadly not all of them at times!
- Be able to demonstrate an understanding that plants produce chemicals that have
antibacterial effects in order to defend themselves, some of which are used by
humans.
- Plants attacked by pathogens can defend
themselves by producing chemicals, often in oil secretions, that have
antibacterial properties.
- Some of these oils have medicinal properties
that humans have used in traditional medicine recipes.
- Other oils have been used as additives in
products of the cosmetics industry.
- Be able to describe how antiseptics can be used to prevent the spread of
infection.
- Antiseptic chemicals are designed to prevent
infection rather than treat and cure an existing infection - prevention is
always better than a cure!
- Antiseptics are chemicals that are applied
to the outside of your body to kill pathogens like bacteria or prevent their
growth.
- Antiseptics help to prevent infection of
cleaned skin wounds and the surface of the skin e.g. a larger area where a
surgical operation might be done and they are also applied to surfaces where
hygiene is important e.g. in the bathroom.
- Antiseptics range from those used in the
home e.g. for cuts and bruises, toilet cleaners, treating food preparation
surfaces, and in GP surgeries, and in hospitals to prevent infection during
operations and on hospital wards to prevent the spread of dangerous
pathogens like MRSA - you should always clean your hands with the antiseptic
facilities provided when visiting friends or relatives in hospital.
- Be able to explain the use of antibiotics to control infection, including:
- Antibiotics are taken internally e.g.
intravenous syringe injection, or orally taken tablet or liquid suspension.
- In other words they are treating you from
the inside and treat an existing pathogen infection you have (bacterial or
fungal microorganism)
- Compare these two point with the external
use of antiseptics in preventing infection.
- a) Antibacterials to treat bacterial infections
- Probably the most well known antibacterial
is the antibiotic penicillin which is effective against many bacterial
infections BUT NOT viruses like the common cold or flue.
- An antibiotic can kill bacteria or prevent
them growing and reproducing.
- b) antifungal to treat fungal
infections
- Antifungal chemicals kill or prevent the
growth of fungi microorganisms e.g creams for the treatment of the fungal
infection athlete's foot.
- Be able to evaluate evidence that resistant strains of bacteria, including
MRSA, can arise from the misuse of antibiotics.
-
Antibiotics, including penicillin, are medicines that
help to cure bacterial disease by killing infectious
bacteria inside the body.
-
What is an antibiotic?
-
Antibiotics cannot be used
to kill viral pathogens, which live and reproduce inside
cells.
-
Antibiotics like
penicillin kill or prevent the growth of harmful pathogens, they kill the
bacteria but not your own body cells.
-
Different antibiotics attack
different bacteria, so it is important that specific bacteria should be
treated by specific antibiotics.
-
The use of antibiotics
has greatly reduced deaths from infectious bacterial
diseases.
-
However, overuse and inappropriate use of antibiotics
has increased the rate of development of antibiotic
resistant strains of bacteria.
-
You need to be aware that it is difficult to develop
drugs that kill viruses without also damaging the body’s
tissues.
-
Many strains of bacteria, including MRSA, have
developed resistance to antibiotics due to mutations, which cause stronger more
resilient strains of bacteria to survive as a result of
natural selection.
-
To prevent further resistance
arising it is important to avoid over-use of antibiotics and only use when
necessary and complete the course of treatment.
-
Knowledge of the development of resistance in bacteria
is limited to the fact that pathogens mutate, producing
resistant strains.
-
Mutations of pathogens produce new strains.
-
Antibiotics and vaccinations may no longer be effective
against a new resistant strain of the pathogen.
-
The new
strain will then spread rapidly because people are not
immune to it and there is no effective treatment.
-
Can bacteria become resistant
to antibiotics?
-
Unfortunately the answer is yes!
Bacteria will sometimes quite naturally mutate into forms that are resistant
to current antibiotics, so if your infected with a new strain of bacteria,
your resistance is not as effective.
-
If an infection is
treated with an antibiotic, any resistant bacteria will survive and
this means resistant bacteria
can survive and reproduce to infect other people, while the non-resistant
strains will tend to be reduced.
-
This is an example of natural
selection at the individual cell level and drug companies are constantly
trying to develop new antibiotics to combat the new evolving strains of
harmful bacteria - but new harmful 'superbugs' are becoming more common the
more we use antibiotics and new epidemics can break out!
-
MRSA, methicillin-resistant
staphylococcus aureus, can't be treated with many current antibiotics and
causes serious wound infections that can be fatal to young babies or elderly
people in particular.
-
Misuse by over-prescribing antibiotics is
believed to be causing the rise of mutant resistant strains of bacteria, so
doctors are being advised to avoid over-prescribing antibiotics to reduce
the mutation rate and not treating mild infections with antibiotics.
-
It isn't just bacteria that can
mutate, viruses can also evolve via new mutations. Viruses are
notable for the rapidity with which they can mutate which makes it difficult
to develop new vaccines. The reason being that changes in the virus (or
bacteria) DNA leads to different gene expression in the form of different
antigens, so different antibodies are needed. The flue virus is a never
ending problem and in the past pandemics (epidemics across many countries at
the same time) have killed millions of people, mercifully this rarely
happens these days thanks to antibiotics.
-
Individual resistant pathogens survive and
reproduce, so the population of the resistant
strain increases.
-
Now, antibiotics are not used to treat
non-serious infections, such as mild throat
infections, so that the rate of development of
resistant strains is slowed down.
- Revise any investigation into the effects of
antiseptics or antibiotics on microbial cultures.
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