Health and Disease

Understanding the difference between health and disease is the starting point for this entire topic. You need to know the World Health Organisation (WHO) definition of health, the distinction between communicable and non-communicable diseases, and how different factors affect the likelihood of developing disease.

What Is Health?

The World Health Organisation (WHO) defines health as:

"A state of complete physical, mental and social well-being, and not merely the absence of disease or infirmity."

This definition is important because it goes beyond just "not being ill." A person who is physically fit but suffering from severe anxiety, or someone who is physically well but socially isolated, would not be considered healthy under this definition.

The Three Dimensions of Health

Dimension	Meaning	Examples of poor health
Physical	The body is free from disease and functioning well	Broken bone, cancer, flu
Mental	The mind is functioning well; emotional well-being	Depression, anxiety, stress
Social	Ability to interact with others and maintain relationships	Isolation, bullying, lack of community

Exam tip: If asked to define health, you must include all three dimensions — physical, mental, and social. Simply writing "not being ill" will not gain full marks.

Types of Disease

Diseases are broadly divided into two categories: communicable and non-communicable.

Communicable Diseases

Caused by pathogens (microorganisms such as bacteria, viruses, fungi and protists)
Can be spread (transmitted) between organisms
Also called infectious diseases
Examples: measles, HIV, tuberculosis (TB), malaria, salmonella

Non-Communicable Diseases (NCDs)

Not caused by pathogens
Cannot be spread between organisms
Often develop over time due to lifestyle, genetics, or environmental factors
Examples: cancer, cardiovascular disease (CVD), type 2 diabetes, asthma, liver disease

Feature	Communicable	Non-communicable
Caused by pathogen?	Yes	No
Can spread between organisms?	Yes	No
Examples	Measles, TB, malaria	Cancer, diabetes, CVD
Prevention focus	Hygiene, vaccination, vector control	Lifestyle changes, screening

Risk Factors for Non-Communicable Diseases

A risk factor is something that increases the likelihood of developing a disease. Risk factors can be:

Lifestyle factors (choices a person makes)
Substances in a person's body or environment
Genetic factors (inherited from parents)

Key Risk Factors

Risk factor	Associated diseases
Smoking	Lung cancer, cardiovascular disease, chronic obstructive pulmonary disease (COPD)
Obesity / poor diet	Type 2 diabetes, cardiovascular disease, some cancers
Excessive alcohol consumption	Liver disease (cirrhosis), liver cancer, brain damage
Lack of exercise	Cardiovascular disease, type 2 diabetes, obesity
Genetic predisposition	Some cancers (e.g. BRCA gene and breast cancer), cystic fibrosis
Ionising radiation	Cancer (mutations in DNA)
Exposure to carcinogens	Cancer (e.g. asbestos → mesothelioma, UV light → skin cancer)

Smoking in Detail

Tobacco smoke contains over 4,000 chemicals, including:

Nicotine — addictive substance that raises heart rate and blood pressure
Tar — carcinogen that coats the lungs and damages alveoli
Carbon monoxide — binds to haemoglobin in red blood cells, reducing oxygen-carrying capacity
Particulates — tiny particles that irritate and damage the airways

Smoking during pregnancy can lead to:

Low birth weight
Premature birth
Stillbirth

Alcohol in Detail

The liver breaks down alcohol. Excessive drinking over time damages liver cells, leading to cirrhosis (scarring of the liver) and eventually liver failure.
Alcohol during pregnancy can cause foetal alcohol syndrome — developmental problems in the baby.

Correlation vs Causation

This is a critical concept that examiners love to test.

Correlation means two variables appear to be linked — when one changes, the other tends to change too.
Causation means one variable directly causes the other to change.

Correlation does not automatically prove causation.

Example

Scientists found a correlation between ice cream sales and drowning rates — both increase in summer. Does ice cream cause drowning? Of course not. The confounding variable is hot weather, which increases both ice cream sales and swimming activity.

How Scientists Establish Causation

Large sample sizes reduce the effect of anomalies
Controlling variables isolates the factor being tested
Peer review — other scientists check the methodology and conclusions
Repeated studies by independent researchers show consistent results
A mechanism is identified (e.g. tar in cigarette smoke damages DNA → uncontrolled cell division → cancer)

Exam tip: If a question gives you a graph showing a correlation and asks "Does this prove that X causes Y?", the answer is almost always no. Explain that correlation does not prove causation and suggest a possible confounding variable.

How Diseases Can Interact

Diseases do not exist in isolation. Having one disease can make a person more susceptible to developing another.

Key Interactions

Situation	Explanation
HIV and communicable diseases	HIV destroys T helper lymphocytes (a type of white blood cell), weakening the immune system. This makes the person much more susceptible to communicable diseases such as TB and pneumonia.
Poor diet and communicable diseases	Malnutrition weakens the immune system because the body lacks the nutrients needed to produce white blood cells and antibodies. This increases vulnerability to infections.
Immune system deficiency and infections	People with defective immune systems (e.g. due to genetic conditions or chemotherapy) are at greater risk of communicable diseases.
Mental health and physical disease	Depression and chronic stress can weaken the immune system (through raised cortisol levels), making infections more likely. Conversely, chronic physical illness can cause or worsen mental health conditions.
Viruses and cancer	Some viruses increase the risk of certain cancers. For example, HPV (human papillomavirus) is linked to cervical cancer. Hepatitis B and C viruses increase the risk of liver cancer.

Exam tip: Be prepared to explain how having one type of disease can increase the risk of developing another type. The most common example in exams is HIV weakening the immune system and leading to increased risk of communicable diseases.

Summary

Health is defined by the WHO as complete physical, mental and social well-being
Communicable diseases are caused by pathogens and can be transmitted
Non-communicable diseases are not caused by pathogens and cannot spread
Risk factors include diet, smoking, alcohol, lack of exercise, and genetics
Correlation does not prove causation — look for confounding variables
Diseases can interact: e.g. HIV weakens the immune system, increasing susceptibility to other infections