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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.
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.
| 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.
Diseases are broadly divided into two categories: communicable and non-communicable.
| 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 |
A risk factor is something that increases the likelihood of developing a disease. Risk factors can be:
| 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) |
Tobacco smoke contains over 4,000 chemicals, including:
Smoking during pregnancy can lead to:
This is a critical concept that examiners love to test.
Correlation does not automatically prove causation.
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.
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.
Diseases do not exist in isolation. Having one disease can make a person more susceptible to developing another.
| 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.