examples explained, transmission, treatments, reduction, prevention
Doc Brown's Biology Revision Notes
Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent
This page will help you answer questions
such as ...
What is a communicable disease?
What is a vector?
How do you prevent the spread of
Describe and explain an example of a
non-communicable diseases and
Introduction to communicable diseases
Communicable diseases are caused by pathogens e.g.
bacteria, fungi, protists or viruses.
Health is the state of an organism's well-being
- physical or mental, but ill health is where there is a problem
including suffering from some disease.
The World Health Organisation defines heath as "a
state of complete physical, mental and social well-being, and not merely
the absence of disease or infirmity".
So, even if you are a very fit person, you are
not necessarily healthy e.g. if you had mental health issues and/or
A disease is a medical condition where part of an
organism (plant or animal) isn't functioning properly - in some way the
organism is not as it should be!
The disease may take the form of cell damage
to the host (plant or animal) which in some way impairs the healthy
('normal') structures or functions of the organism.
organisms, including ourselves, experience ill health at some point
in their life.
If you have an increased chance of contracting a
disease you are described as susceptible.
There are many causes of ill health in plants
and animals e.g.
infection from a pathogen eg flue,
mutation in an organism's genes (DNA)
an organism might suffer some deficiency
eg lack of vitamins in human diet, lack of light on plant growth
an organism may experience mental or
physical trauma triggered by some event eg depression,
bereavement, serious accident,
the lifestyle of an organism can have
consequences on your health eg links between: smoking and lung
cancer, too much sugary/fatty food and obesity and/or diabetes,
All diseases show symptoms at some point in
Symptoms are indications of disease in an
organism - usually observable eg cough, rash, diarrhoea, leaf
Sometimes symptoms do not show up immediately
after infection - the virus or bacteria may multiply for days or
weeks when sufficient of the pathogen is present to create visible
This period of infection without symptoms
is called the incubation period and may last hours, days,
weeks or months - which is a bit scary, because you can't apply
medical treatment to a medical condition you don't know you've
There are two sorts of diseases - communicable and
Communicable diseases that are spread
between individual organisms - animals and people or person to person.
Communicable diseases are also known as infectious diseases.
They can be caused by bacteria, fungi,
parasites or viruses.
They can be sometimes described as contagious
or infectious diseases.
Examples are diseases like malaria or measles.
Non-communicable diseases cannot be
transmitted between individual organisms e.g. cancer, diabetes, heart
diseases (eg cardiovascular) or respiratory diseases of the lung.
See separate page on
Note: When you have one medical condition you
may be more susceptible to a communicable disease.
If you are suffering from one disease, your bodies
defences may be weakened by it making you more susceptible to another
disease - a 'knock on' effect reducing your body's ability to fight off
a second disease e.g.
People with problems with their immune system by which your body
defends itself against infections, may be far more susceptible to
other communicable diseases such as influenza. The body is less able
to fight off the infection from particular pathogens like the flue
Lifestyle choices and your personal situation
Eating a good balanced healthy diet helps maintain your
body in good shape and your immune system to fight communicable
disease infections and reduce the risk of contracting communicable
diseases. The poorer you diet, the more susceptible you are to
infection by pathogens.
Access to your needs?
Usually no problem in rich developed countries - your choices,
BUT not so for people living in poorer underdeveloped countries.
e.g. to help prevent or reduce the risk of communicable diseases
in the first place - an you afford to buy healthy food? Is
'healthy shopping' readily available?
If you have contracted a communicable disease, do you have access
to appropriate medicines?
Do you have access to contraception e.g. condoms to prevent the
transmission of sexually transmitted disease.
Mental health and stress
If can develop a mental health condition such as depression while
enduring some physical health problem e.g. lack of mobility reducing
your ability to participate fully in everyday life. If you are
constantly under mental stress e.g. 'high-powered' job or caring for
a very sick relative, then your physical well-being can be affected
- ulcers can develop or a mental health condition like extreme
of pathogens causes communicable
Communicable diseases are caused by pathogens - types of
microorganisms that enter an organism e.g. your body, and cause disease.
Pathogens cause diseases in both plants and animals.
Types of pathogen
Bacteria are very
small cells, compared to your own body cells, which can rapidly reproduce by cell division in your body.
They make you feel ill by
damaging your body's cells and tissues and producing toxins - poisons produced as a
by-product of the bacteria's cell chemistry that can damage your cells
Viruses (See also
Viruses are NOT
cells, they are much smaller than bacteria and damage the cells in which
Viruses rapidly replicate by invading a
cell and using the cell's genetic machinery to reproduce themselves ie
copies of the original virus.
The virus 'invaded' cell then
bursts releasing lots of new viruses to go and invade other cells.
The cell damage makes you feel
ill as your body (temporarily) fights back to make as many good cells as it
can to replace those destroyed by the virus.
Protists are usually single celled eukaryotes
and there are many types and sizes of them.
Some eukaryotic protists are parasites and live on or inside the 'host'
organism causing some kind of damage.
They are usually transferred to an organism by a vector
which isn't affected by the disease itself e.g. an insect
Fungi can be single celled or
others have a 'body' consisting of multi-celled
thread-like structure called hyphae.
Hyphae can grow and penetrate human skin and the
surface of plants causing damage.
Hyphae can produce spores that spread to other plants
Larger organisms - some nasty species
Helminths are a type of parasitic worm
that can get inside your body e.g. tapeworms, flukes, and
roundworms. Helminthiasis, also known as worm infection, is any
macroparasitic disease of humans and other animals in which a
part of the body is infected with parasitic worms, known as
helminths. Soil-transmitted helminthiases are responsible for
parasitic infections in as much as a quarter of the human
population worldwide. They can cause damage to the intestine
wall, inflammation, damage organs such as the skin, lungs and
live and also cause neurological problems.
Trichinosis is a disease caused by
trichinae helminth, typically from infected meat,
characterized by digestive disturbance, fever, and muscular
Ironically, it is thought that
infection by trichinae may reduce the development of some
autoimmune diseases like Crohn's disease.
Autoimmune diseases are diseases
caused by the body's immune system treating its own cells as
if they are foreign and attacking them.
How are pathogens spread?
There many sorts of ways that pathogens can spread.
Pathogens are carried in air currents and breathed in
e.g. fungal spores.
Airborne pathogens are conveyed through the air in water
droplets when we cough or sneeze e.g. the influenza virus
giving us 'flue'.
Pathogens can be picked up by merely touching a contaminated
surface including someone's skin e.g. athlete's foot is a
fungus which makes skin itch and flake off. It is most
commonly spreading by coming into contact with a surface an
infected person has e.g. shower floors or towels.
Dirty contaminated water is a common source of pathogens and
should not be drank or bathed in. Cholera is a bacterial
infection spread by drinking water contaminated with the
diarrhoea of people already infected with the bacteria.
Pathogens can be spread through bodily
fluids like blood (drug users sharing needles), breast feeding
milk, and sexual activity (contact with semen).
is good example.
The mosquito is an example of an animal
vector which carries the protist pathogen that causes
plant disease notes - pathogens
in the soil
Contaminated food may contain the
salmonella bacterium causing food poisoning.
Examples of communicable
The disease, pathogen, symptoms-effects, means of spread and how to
reduce or prevent transmission are all described in the following examples.
of communicable diseases
Salmonella is a type of bacteria that causes food poisoning
e.g. from contaminated food.
The symptoms experienced by infected people include diarrhoea,
fever, stomach cramps and vomiting.
These very unpleasant effects are caused by toxins produced by
the salmonella bacteria.
Salmonella food poisoning is often contracted from animals like
chickens catching the disease when alive - if 'undercooked' the
salmonella bacterium are not killed.
Salmonella can also be contracted by eating food prepared in
unhygienic conditions and food that has been stored too long at too
high a temperature - allowing salmonella bacteria to multiply.
In the UK, to control the spread of the disease, poultry like
chickens and turkeys are given a vaccination against salmonella. In
the US chickens are washed with a chlorine solution to kill the
Whatever the poultry source, make sure it is well cooked!
Gonorrhoea is a bacterium that is classed as a sexually
transmitted disease (STD).
(sexually transmitted infection, STI)
Sexually transmitted diseases are passed on by sexual contact
e.g. having unprotected sex.
A person infected with gonorrhoea bacteria will experience pain
on urination and other symptoms include a thick yellow or green
discharge from the penis or vagina.
Gonorrhoea was, and still is, treated with the antibiotic
penicillin, but unfortunately, strains of bacteria have evolved that
resist this treatment, making it less effective
The best strategy to reduce the spread of gonorrhoea is to treat
patients with new antibiotics that the bacteria are not as resistant
too AND to use barrier methods of contraception like condoms.
Cholera is from a bacterium called Vibrio cholerae.
Cholera bacteria cause diarrhoea and in severe cases causes
dangerous fluid loss - dehydration, which can be fatal.
The principal source is contaminated water supply containing the
cholera bacterium, which is the means by which the cholera bacteria
Poor hygiene further contributes because the
faeces contain the cholera bacteria too.
Therefore, prevention is best obtained by using a clean water
supply - there were outbreaks of cholera in Victorian England until
it was realised the source of the infection was dirty water!
Tuberculosis is from a bacterium called Mycobacterium
The effects of the tuberculosis bacteria include coughing and
lung damage - older people can be severely weakened and further
medical complications may arise.
The bacteria are conveyed through the air when infected people
cough - the principal symptom!
Infected people minimise contact with people, practice good
hygiene - like coughing into a tissue, sleep alone and keep the
house well ventilated.
Stomach ulcers can be caused by the bacterium Helicobacter
Stomach ulcer symptoms and effects include stomach pain, nausea
The bacteria reduce the stomach's defences against the acid it
produces to digest food.
The bacteria are ingested after eating contaminated food or
drinking contaminated water - referred to as an example of oral
The chances of getting a stomach ulcers are reduced by having
access to clean water and watching your personal hygiene e.g.
washing hands (toilet, food preparation) and a clean house!
Treatments for stomach ulcers include the use of antibiotics and
drugs that reduce stomach acid production.
Chlamydia is a kind bacterium that is classed as a sexually
transmitted disease (STD).
(sexually transmitted infection, STI)
The chlamydia bacteria behave like a virus, because it can only
reproduce inside a living host cell.
Sexually transmitted diseases are passed on by sexual contact
e.g. having unprotected sex.
Chlamydia doesn't always produce symptoms BUT it can cause
infertility in men and women.
The spread of chlamydia can be reduced by ...
(i) wearing a condom during sexual intercourse,
(ii) screening individuals so that they can be treated with
(iii) refraining from sexual contact.
Escherichia coli (E. coli)
E.coli bacteria normally live in the intestines of healthy people
and animals and are not usually a problem.
varieties of E. coli are
harmless or cause relatively brief diarrhea, there are a few
particularly nasty strains which can cause severe abdominal cramps,
bloody diarrhea and vomiting.
E.coli cells can divide every 20 minutes, so their numbers can
Examples of viral pathogens of communicable diseases
Measles is a viral disease spread by droplets when an infected
person sneezes or coughs.
The primary symptoms are a red skin rash and a higher than normal
body temperature from a fever.
Measles can be a serious medical condition and even fatal if
complications develop e.g. a lung infection like pneumonia or the
brain infection encephalitis.
Most people are adequately protected if you are vaccinated when
Human Immunodeficiency Virus (HIV) is a virus spread by sexual contact or by exchanging
bodily fluids like blood e.g. when two people share the same needle in
drug taking, sexual intercourse - semen and vaginal fluids.
Prevention of infection and minimising the
spread of HIV ..
(i) Using a condom during sexual
(ii) Drug users NOT sharing needles,
(iii) There are some medications (antiretroviral
drugs) available to reduce the risk of
passing on the infection during sex - this also applies to
mothers passing on the infection to babies during pregnancy.
(iv) Being screened and following up with
suitable treatment e.g.
drugs which stop the virus reproducing.
Usually, initially, the HIV infected person experiences flue-like
symptoms for a few weeks but then no other symptoms may be
experienced for several years.
The virus enters the lymph nodes and attacks
some of the white blood cells of the immune system.
If diagnosed in time, HIV can be controlled with antiretroviral
drugs that stop the HIV virus replicating in the body.
The HIV virus attacks some of the types of white blood cells,
kills them and
so damaging part of the bodies immune system.
This means the body's defences against other infections,
the immune system, is
severely weakened and may not be able to cope, including an
increased risk of cancer.
At this advanced stage, when your body is struggling to cope
with any infection at all, HIV leads to 'late stage HIV infection' known as
Acquired Immune Deficiency Syndrome (AIDS).
Since HIV prevents the immune system from
working properly, the body is extremely vulnerable to
infection from any other pathogen - an unfortunate 'knock
e.g. the bacteria that cause the
tuberculosis would normally
be destroyed by the body's immune system before symptoms
develop. However, the immune system of someone infected with the
HIV virus are much more likely to display symptoms of
tuberculosis and the disease may develop very rapidly - recovery
is much more difficult and the outcome can be serious e.g.
persistent coughing and lung damage.
Tobacco mosaic virus (TMV) is a virus that attacks plants. See
Plant Diseases notes for
Ebola is caused by a virus which causes a fever accompanied by
bleeding (haemorrhagic fever).
Ebola spreads through human-to-human transmission via direct
contact (through broken skin or mucous membranes) with the blood,
secretions, organs or other bodily fluids of infected people, and
with surfaces and materials (e.g. bedding, clothing) contaminated
with these fluids.
To reduce the spread of Ebola, infected people should be isolated
and sterilising any areas where the virus might be.
Human papillomavirus (HPV) is a virus that
can infect the human reproductive system.
The HPV virus is transmitted in bodily fluids,
usually during sexual activity.
Infection by the HPV virus doesn't always show
symptoms and usually clears up on its own in a couple of months.
Unfortunately, sometimes the HPV infection
promotes cell DNA changes causing the formation of certain types of
It is thought that most cases of cervical
cancer arise from HPV infections - one disease causes another.
In this case of a communicable disease (HPV)
causes the formation of a non-communicable disease (cancer).
Chicken pox is caused by a virus - typical
symptom is a spotty rash.
When a person has become infected with this
pathogen, it takes nearly 14 days for the rash to appear, this time
is called the incubation period.
The hepatitis virus causes long-term infections in the liver
where it lives in the cells.
This gives you an increased chance of developing liver cancer - a
case of a communicable disease making you more susceptible to a
of communicable diseases
Malaria is caused by a protist (a type of eukaryote cell).
Part of the malarial protist's life cycle is inside a mosquito
insect - which feeds by sucking blood from warm-blooded animals
The mosquito is an animal vector - a 'carrier' - a means
of spreading a disease - it conveys the protist without any
ill-effect to itself - it just passes the protist on without
developing the disease!
The mosquitos pick up the malarial protist when they feed on an
When the mosquito feeds on another animal, it infects it by
inadvertently passing the malarial protist into the blood stream
Malaria causes damage to red blood cells and in severe cases, the
Malaria causes repeated episodes of fever which can overwhelm the
bodies defences and prove fatal.
Malaria is still a common infection in many parts of the world
but you can do some things to combat it.
You can reduce the spread of malaria by stopping the
mosquitos from breeding e.g. affecting their breeding grounds by
For your own personal protection you can use insecticide
sprays (to kill the mosquitos or act as a repellent) and
mosquito nets - all of which are designed to stop mosquitos
biting you and passing the disease on!
Rose black spot is fungus that affects rose plants. See
notes for more details.
Athlete's foot is a fungus which makes skin itch and
It is most commonly spreading by a person coming into contact
with a surface touched by an infected person e.g. shower floors or
Chalara ash dieback is a fungus that infects ash trees. See
notes for more details.
How can we prevent, or reduce
the spread of communicable diseases
You should be as hygienic as possible in your everyday life to
reduce the spread of communicable diseases e.g.
washing your hands thoroughly after going to the toilet,
washing your hands before preparing and handling food, sneezing
into a tissue rather than into the 'open air' and disposing of
If you are infected with a disease, as far is practicable, try to
minimise contact with other people and therefore minimise passing
the infection on - in extreme cases, you might end up in an
Where possible, vaccination of people or animals can be used to
control the spread of communicable diseases.
Vaccination prevents the disease from developing and so the
infection cannot be passed on.
If you can kill or reduce the organisms that carry a disease (the
vectors) you minimise their ability to pass the disease on.
Vectors like insects can be killed using insecticides or
destroying their breeding ground habitats.
XREF healthy 2
VIRUSES - reproduction
What are viruses?
Viruses are NOT cells, but usually consist of a strand of genetic
material enclosed in a protein coat.
How do they reproduce?
They cannot reproduce on their and have to 'invade' i.e. infect a
living cell that acts as a host.
Specific types of viruses only infect specific cells and
persuades them to reproduce the invading virus.
The life cycle of a virus
The life cycle of a virus begins when it gets through a cell
membrane to infect a host cell.
Most viruses reproduce by the lytic pathway and but others
involve the lysogenic pathway first.
The lytic pathway
In the lytic pathway the virus attaches itself to a specific
host cell and injects its genetic material (viral DNA)
into the cytoplasm through the cell membrane.
The virus then uses the proteins and enzymes of the host cell
to replicate its genetic material (DNA instructions) and
so produce the material for the new viruses.
The virus components then assemble to form lots of new
viruses causing the cell to split open - thus releasing lots
more viruses to invade more cells to create more viruses!
The lysogenic pathway
In the lysogenic pathway the injected viral genetic material
is incorporated into the DNA genome of the host cell.
The viral genetic material gets replicated along with the
host DNA every time the host cell divides, but for the time
being, the virus stays dormant - doesn't do anything - so no new
viruses are formed.
Eventually, some kind or trigger e.g. from a chemical
stimulation, causes the viral genetic material to leave the
genome and then proceed to reproduce using the lytic pathway
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