What you'll learn
- What the WHO means by health, and how this differs from simply “not being ill”.
- The difference between communicable and non-communicable diseases.
- What pathogens are, including viruses, bacteria, fungi and protists.
- How common infections spread, and how we can reduce or prevent that spread.
Health is more than “not ill”
The World Health Organization (WHO) defines health as a state of complete physical, mental and social well-being — not just the absence of disease or infirmity.
Health
Health is a state of complete physical, mental and social well-being, not merely the absence of disease or infirmity.
The three parts of health
- Physical well-being means your body is functioning properly, for example your lungs, heart and immune system.
- Mental well-being means your emotional and psychological state, such as coping with stress.
- Social well-being means your ability to form relationships and take part in society, such as school, work and family life.
A person can have no diagnosed disease but still have poor health, for example if they are extremely stressed, isolated or unable to carry out normal daily activities.
Big idea
Health is about overall well-being. Disease is one possible cause of poor health, but it is not the only one.
Diseases: communicable and non-communicable
A disease is a condition that affects the normal functioning of the body or part of the body.
Communicable disease
A communicable disease is a disease that can be passed from one organism to another.
Communicable diseases are often caused by pathogens, such as bacteria, viruses, fungi or protists. Examples include cholera, tuberculosis, malaria and HIV.
Non-communicable disease
A non-communicable disease is a disease that cannot be passed from one organism to another.
Examples include cancer, coronary heart disease and diabetes. These can be linked to genetics, lifestyle or environmental factors, but they are not “caught” from another person.
Infectious does not mean inherited
Communicable diseases are spread between organisms. Inherited conditions are passed from parents to offspring through genes, which is a different idea.
Classifying diseases
A doctor studies two conditions. Condition A is caused by a bacterium and spreads when people drink contaminated water. Condition B is caused by long-term damage to blood vessels and cannot be passed between people.
- Condition A is caused by a pathogen and can spread from person to person through water, so it is communicable.
- Condition B cannot be passed between people, so it is non-communicable.
- The key comparison is whether the disease can be transmitted from one organism to another.
One disease can increase susceptibility to another
Susceptibility means how likely an organism is to become infected or affected by a disease.
If someone already has one disease, their body may be less able to defend itself against another disease. This can happen because:
- the first disease damages tissues, making it easier for pathogens to enter
- the first disease weakens the immune system
- the person becomes malnourished or exhausted, reducing their ability to recover
- treatment for one disease may sometimes affect the body’s defences
A key GCSE example is HIV, which destroys certain white blood cells. These cells are important in the immune response, so a person with advanced HIV infection is more likely to develop other infections. When the immune system becomes badly damaged, this can lead to AIDS.
AIDS
AIDS is the late stage of HIV infection, when the immune system is severely damaged and the person becomes vulnerable to other infections and diseases.
Linking HIV to higher infection risk
A patient has HIV and later develops tuberculosis.
- HIV infects and destroys some white blood cells, which normally help defend the body against pathogens.
- With fewer functioning white blood cells, the immune system is less effective at detecting and attacking tuberculosis bacteria.
- The patient is therefore more susceptible to tuberculosis because their immune defences have been weakened.
Pathogens cause communicable disease
Pathogen
A pathogen is a disease-causing organism or infectious agent.
At GCSE, you need to know four main groups of pathogens.
Viruses
Viruses are very small infectious particles. They can only reproduce inside living host cells. HIV and Ebola are examples of viral infections.
Bacteria
Bacteria are single-celled organisms. Some bacteria are harmless or useful, but pathogenic bacteria can cause disease. Cholera, tuberculosis, Chlamydia and Helicobacter infections are caused by bacteria.
Fungi
Fungi include moulds, yeasts and some plant pathogens. Chalara ash dieback is caused by a fungus and affects ash trees.
Protists
Protists are single-celled eukaryotic organisms. Malaria is caused by a protist.
Remember the four pathogen groups
A quick GCSE list is viruses, bacteria, fungi and protists. Not all microorganisms are pathogens — only the ones that cause disease.
Common infections you need to know
Here are the main examples from this topic.
| Infection | Pathogen type | Main effect |
|---|---|---|
| Cholera | Bacterium | Causes diarrhoea |
| Tuberculosis, often shortened to TB | Bacterium | Causes lung damage |
| Chalara ash dieback | Fungus | Causes leaf loss and bark lesions in ash trees |
| Malaria | Protist | Causes damage to blood and liver |
| HIV | Virus | Destroys white blood cells, leading to AIDS |
| Stomach ulcers | Bacterium called Helicobacter | Damages the stomach lining |
| Ebola | Virus | Causes haemorrhagic fever |
The stomach ulcer and Ebola examples are included for Biology students on the separate GCSE course, such as 1BI0.
Haemorrhagic fever
Haemorrhagic fever is a severe illness involving fever and bleeding, because blood vessels and organs are damaged.
How pathogens spread
Pathogens must move from an infected organism or contaminated source to a new host. Different diseases spread in different ways, so the best prevention method depends on the transmission route.
Water: cholera
Cholera bacteria spread through contaminated water. If sewage enters drinking water, people may swallow the bacteria and develop severe diarrhoea.
Spread can be reduced by:
- clean drinking water
- good sanitation
- treating sewage properly
- washing hands
Airborne droplets: tuberculosis
Tuberculosis bacteria spread in airborne droplets when an infected person coughs or sneezes. Another person can breathe in the droplets.
Spread can be reduced by:
- covering coughs and sneezes
- good ventilation
- isolating infected people when needed
- vaccination where appropriate
- treating infected people with antibiotics
Airborne spores: Chalara ash dieback
Chalara ash dieback spreads by fungal spores carried in the air. Spores can infect ash trees, causing leaf loss and bark lesions.
Spread can be reduced by:
- removing infected plant material
- controlling movement of infected wood or plants
- cleaning boots, tools and vehicles after visiting affected woodland
Animal vectors: malaria
Vector
A vector is an organism that spreads a pathogen from one host to another, without causing the disease itself.
Malaria is spread by mosquitoes. The mosquito is the vector. When an infected mosquito feeds on a human, the malaria protist can enter the bloodstream.
Spread can be reduced by:
- mosquito nets
- insecticides
- draining standing water where mosquitoes breed
- preventing mosquito bites
Oral transmission: Helicobacter
Some stomach ulcers are caused by Helicobacter bacteria. In Biology, you need to know that these bacteria can spread by oral transmission, for example through saliva or contaminated food and water.
Body fluids: Ebola
Ebola spreads through contact with body fluids from infected people, such as blood, vomit or diarrhoea.
Spread can be reduced by:
- isolating infected patients
- wearing personal protective equipment, often called PPE
- safe handling of body fluids
- safe burial practices
- careful cleaning and disinfection
Choosing prevention methods
A village has many cases of cholera after flooding damages the sewage system.
- Cholera is caused by bacteria that spread through contaminated water, so the likely source is drinking water mixed with sewage.
- The most direct prevention methods are to provide clean water, repair sanitation and treat sewage.
- Methods aimed at airborne spread, such as improving ventilation, would not target the main transmission route for cholera.
Using the wrong prevention for the wrong route
Do not just write “wash hands” for every disease. Link your prevention method to the route of spread: water, air, vector, sexual contact, oral transmission or body fluids.
Sexually transmitted infections: Chlamydia and HIV
A sexually transmitted infection, or STI, is an infection spread mainly through sexual contact.
Chlamydia is caused by bacteria. HIV is caused by a virus. Both can spread during sexual contact if infected body fluids pass between people.
Spread can be reduced by:
- using condoms
- STI testing
- treating infected people where possible
- not sharing needles, especially for HIV
- informing sexual partners so they can be tested
Chlamydia can often be treated with antibiotics because it is bacterial. HIV cannot be cured by antibiotics because it is viral, but antiviral drugs can reduce viral replication and help control the infection.
Antibiotics and viruses
Antibiotics kill bacteria or stop them growing. They do not kill viruses, so they would not cure HIV.
Virus lifecycle: lytic and lysogenic pathways
For Biology students on the separate GCSE course, you also need to describe the lifecycle of a virus, including the lytic and lysogenic pathways.
Viruses cannot reproduce on their own. They must enter a host cell and use the host cell’s machinery to make more viruses.
The lytic pathway
Lytic pathway
The lytic pathway is a viral lifecycle in which the virus quickly replicates inside a host cell, then the host cell bursts and releases new viruses.
In the lytic pathway:
- The virus attaches to a host cell.
- Viral genetic material enters the host cell.
- The host cell makes viral genetic material and proteins.
- New virus particles are assembled.
- The host cell bursts, releasing many new viruses.
The bursting of the cell is called lysis.
The lysogenic pathway
Lysogenic pathway
The lysogenic pathway is a viral lifecycle in which viral genetic material becomes inserted into the host cell’s DNA and may stay inactive for some time.
In the lysogenic pathway, the virus does not immediately destroy the cell. Instead, its genetic material is copied when the host cell divides. Later, it can become active and enter the lytic pathway.
Lytic versus lysogenic
The lytic pathway makes new viruses straight away and destroys the host cell. The lysogenic pathway hides viral genetic material in the host DNA before it may later become active.
Comparing viral pathways
A virus enters a cell. In one infection, the host cell bursts after making many new viruses. In another infection, the viral DNA is copied with the host DNA for many cell divisions before becoming active.
- The infection where the cell bursts is the lytic pathway, because new viruses are made immediately and released by lysis.
- The infection where viral DNA is copied with host DNA is the lysogenic pathway, because the virus remains inactive inside the host genome.
- If the inactive viral DNA later starts producing new viruses, it has switched from the lysogenic pathway into the lytic pathway.
In the exam
- Link each disease to its pathogen type, its effect, and its route of spread.
- For prevention questions, choose a method that matches the route of transmission, such as clean water for cholera or mosquito nets for malaria.
- If asked why one disease increases risk of another, explain the mechanism: tissue damage, weakened immune system or reduced white blood cell function.
Check yourself
- What is the difference between a communicable and a non-communicable disease?
- Why does HIV make a person more susceptible to other infections?
- For cholera, tuberculosis and malaria, can you name the pathogen type, route of spread and one prevention method?
