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Communicable diseases: 1.  Introduction to communicable diseases

Doc Brown's biology exam revision study notes

There are various sections to work through, after 1 they can be read and studied in any order.

Sub-index of biology notes on communicable diseases

(1) Introduction to communicable diseases (transmissible diseases/infections)

Communicable/transmissible diseases are usually caused by pathogens e.g. bacteria, fungi, protists or viruses.

Communicable/transmissible diseases are those in which the pathogen passes from one host to another i.e. one organism to another, plant or animal.

(A non-communicable/non-transmissible disease cannot be passed from one host to another.)

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 lonely.

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.

Most 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 e.g. flue, malaria, salmonella,

mutation in an organism's genes (DNA) eg cancers,

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 their development.

Symptoms are indications of disease in an organism - usually observable eg cough, rash, diarrhoea, leaf discolouration etc.

Sometimes symptoms do not show up immediately after infection - the virus or bacteria may multiply for days or weeks until sufficient of the pathogen is present to create visible symptoms.

After a pathogen has entered an organism (infection), there is a period of infection without symptoms 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 got!

It is in the incubation period that harmful toxins build up.

The more pathogen present (bacteria or virus) the more rapidly toxins build up and you then experience typical symptoms like headache, raised temperature, stomach discomfort - fever etc.


Diseases can be classified as communicable and non-communicable.

Communicable diseases that are spread between individual organisms - animals and people or person to person.

Because communicable diseases can spread between organisms (plants or animals), they are often described as contagious or infectious diseases.

They can be caused by bacteria, fungi, parasites or viruses.

Examples are diseases like malaria, tuberculosis 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 non-communicable diseases 

When you have one medical condition e.g. a communicable disease, you may be more susceptible to another 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 virus.

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 weaker your immune system is, by not accessing the correct balance of nutrients, therefore you are more susceptible you are to infection by pathogens.

Access to your needs?

The risk of infection from a communicable disease increases if you have limited access to good healthcare systems and health education.

When you have access to a quality healthcare system, your medical condition is more likely to be diagnosed and receive appropriate treatment. In turn this also reduces the chance of you passing on the infection.

Education provides you with knowledge about how diseases are transmitted and help avoid infection in the first place - see examples down the page on HIV and safe sex practice.

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 anxiety.

Keywords, phrases and learning objectives for this part on communicable diseases

Know what is meant by a communicable diseases and a transmissible infection?

Be able to describe and explain examples of the causes and characteristics of communicable diseases.



INDEX of notes on communicable diseases


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