Communicable Diseases

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 communicable diseases?

 Describe and explain an example of a communicable disease?

See also non-communicable diseases  and  plant diseases

and Keeping healthy - our defences against pathogens, fighting infectious diseases, vaccination, monoclonal antibodies

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 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 eg 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 when sufficient of the pathogen is present to create visible symptoms.

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 got!

Diseases can be classified as communicable and non-communicable.

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 non-communicable diseases 

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 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.

Which types of pathogens causes communicable diseases?

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 and tissues.

Viruses  (See also Appendix on viruses)

Viruses are NOT cells, they are much smaller than bacteria and damage the cells in which they reproduce.

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 carrying protist.


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 and animals.

Larger organisms - some nasty species out there!

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 rigidity.

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'.

When people are crowded together, their close proximity allows an easy transfer of a communicable disease from one person to another e.g. coughing out a cold or flue virus!

Direct contact

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.

Body fluids

Pathogens can be spread through bodily fluids like blood (drug users sharing needles), breast feeding milk, and sexual activity (contact with semen). HIV is good example.

Animal vectors

The mosquito is an example of an animal vector which carries the protist pathogen that causes malaria.


See plant disease notes - pathogens in the soil


Contaminated food may contain the salmonella bacterium causing food poisoning.

Examples of communicable diseases

The disease, pathogen, symptoms-effects, means of spread and how to reduce or prevent transmission are all described in the following examples.

Examples of bacterial pathogen 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 salmonella bacterium.

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 are spread.

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 tuberculosis

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 pylori

Stomach ulcer symptoms and effects include stomach pain, nausea and vomiting.

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 transmission.

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 oral antibiotics,

(iii) refraining from sexual contact.

Escherichia coli (Ecoli) bacterium

E.coli bacteria normally live in the intestines of healthy people and animals and are not usually a problem.

However, while most varieties of Ecoli 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 rapidly increase!


Examples of viral pathogen 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 young.

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 intercourse

(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. antiretroviral 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 on' effect.

e.g. the bacteria that cause the communicable disease 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 more details.

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 cancer.

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 non-communicable disease.


Examples of protist pathogen 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 (including us!).

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 infected animal.

When the mosquito feeds on another animal, it infects it by inadvertently passing the malarial protist into the blood stream when feeding.

Malaria causes damage to red blood cells and in severe cases, the liver.

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 draining swamps.

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!

Examples of fungal pathogen communicable diseases

Rose black spot is fungus that affects rose plants. See Plant Diseases notes for more details.

Athlete's foot is a fungus which makes skin itch and flake off.

It is most commonly spreading by a person coming into contact with a surface touched by an infected person e.g. shower floors or towels.

Chalara ash dieback is a fungus that infects ash trees. See Plant Diseases notes for more details.


How can we prevent, or reduce the spread of communicable diseases

Early detection and treatment

This reduces chances of spreading a disease, but it is very dependant on your own knowledge of a disease and access to a good healthcare system for diagnosis and treatment.


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 it.

Treating and cleaning kitchen worktops with antiseptic sprays.


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 isolation ward.


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.

See Keeping healthy - defence against pathogens, infectious diseases, vaccination for more details


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.

Isolating yourself, as far as is practicable, from other people, reducing your chances of passing on your infection. Some people with a serious communicable disease may be placed in a hospital isolation ward.

Appendix 1 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 described above.


General HUMAN BIOLOGY revision notes

See also cell biology index above

Introduction to the organisation of cells => tissues => organs => organ systems (e.g. in humans)

Examples of surfaces for the exchange of substances in animal organisms   gcse biology revision notes

See also Enzymes - section on digestion and synthesis  gcse biology revision notes

The human circulatory system - heart, lungs, blood, blood vessels, causes/treatment of cardiovascular disease

Homeostasis - introduction to how it functions (negative feedback systems explained)  gcse biology revision notes

Homeostasis - control of blood sugar level - insulin and diabetes  gcse biology revision notes

Homeostasis - osmoregulation, ADH, water control, urea and ion concentrations and kidney function, dialysis

Homeostasis - thermoregulation, control of temperature  gcse biology revision notes

The brain - what the different parts do and the dangers if damaged gcse biology revision notes

An introduction to the nervous system including the reflex arc  gcse biology revision notes

Hormone systems - Introduction to the endocrine system - adrenaline & thyroxine hormones  gcse biology revision

Hormone systems - menstrual cycle, contraception, fertility treatments  gcse biology revision notes

Respiration - aerobic and anaerobic in plants and animals.  gcse biology revision notes

Keeping healthy - communicable diseases - pathogen infections   gcse biology revision notes

Keeping healthy - non-communicable diseases - risk factors for e.g. cancers   gcse biology revision notes

Keeping healthy - diet and exercise  gcse biology revision notes

Keeping healthy - defence against pathogens, infectious diseases, vaccination, drugs, monoclonal antibodies

See also Culturing microorganisms like bacteria - testing antibiotics/antiseptics  gcse biology revision

Food tests for reducing sugars, starch, proteins and lipids  gcse biology revision notes

The eye - structure and function - correction of vision defects  gcse biology revision notes

Optics - lens types (convex, concave, uses), experiments, ray diagrams, correction of eye defects (gcse physics)

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