B1: Epidemiology Study Guide

1. What Is Epidemiology?

Epidemiology is the study of how diseases (and other health-related events) are distributed in populations and the factors that influence or determine this distribution.

• Looks at who is affected, where, and when.
• Helps identify risk factors and evaluate strategies to prevent or control disease.

2. Key Epidemiological Terms

• Health – A state of complete physical, mental, and social well-being, not merely the absence of disease.
• Disease – Any condition that impairs normal functioning of the body or mind.
• Communicable disease – A disease that can be passed from one person (or organism) to another (e.g. flu, COVID-19, measles).
• Non-communicable disease – A disease that is not passed directly between people (e.g. heart disease, diabetes, cancer).
• Agent – The microorganism or factor that causes disease (e.g. bacteria, virus).
• Host – The organism (often a human) that can be infected by the agent.
• Environment – External factors (e.g. climate, housing, sanitation) that affect disease transmission.
• Vector – A living organism that transmits a pathogen (e.g. mosquito, tick).
• Fomite – A non-living object that can carry infectious organisms (e.g. door handle, phone, towel).
• Reservoir – The place where a pathogen normally lives and multiplies (e.g. humans, animals, water, soil).

3. Measures of Disease Frequency

Epidemiologists measure how often diseases occur using incidence and prevalence.

• Incidence – The number of new cases of a disease that develop in a particular population during a specific time period.
  Example: “There were 50 new cases of measles in the city in 2025.”
• Prevalence – The total number of existing cases (new and old) in a population at a given time.
  Example: “At the start of 2025, 200 people in the city were living with asthma.”
• Morbidity – Illness or disease in a population.
• Mortality – Deaths in a population, often expressed as a mortality rate.

4. Patterns of Disease: Endemic, Epidemic, Pandemic

• Endemic – A disease that is constantly present at a low or moderate level in a particular area or population.
  Example: Malaria is endemic in some tropical regions.
• Epidemic – A sudden increase in the number of cases of a disease above what is normally expected in a population or area.
  Example: An outbreak of measles in a school that spreads quickly is an epidemic.
• Pandemic – An epidemic that spreads across many countries or continents, usually affecting a large number of people.
  Example: COVID-19 became a global pandemic in 2020.

Tip to remember:
Endemic = “always there” at some level.
Epidemic = “extra cases” in one region.
Pandemic = “across many countries”.

5. Modes of Disease Transmission

Direct transmission

• Direct contact – Person-to-person physical contact (e.g. touching, kissing, sexual contact).
• Droplet spread – Large droplets from coughs/sneezes that travel a short distance.

Indirect transmission

• Airborne – Very small particles or droplets that remain in the air over time and distance.
• Vehicle-borne – Contaminated water, food, blood, or other substances.
• Vector-borne – Transmitted by vectors such as mosquitoes or ticks.
• Fomite-borne – Transmitted via contaminated surfaces or objects.

Prevention links:
– Handwashing & masks: reduce direct contact and droplet spread.
– Safe water & food handling: reduce vehicle-borne transmission.
– Insect control (nets, repellents): reduce vector-borne transmission.

6. Types of Epidemiological Studies

• Descriptive studies – Describe who, where, and when (person–place–time). Often the first step to identify patterns.
• Analytical studies – Look for causes or risk factors.
  • Cohort study – A group of people is followed over time to see who develops the disease. Compares exposed vs. unexposed groups.
  • Case-control study – Starts with people who already have the disease (cases) and compares them to people without the disease (controls) to look back at possible risk factors.
• Experimental (intervention) studies – The researcher actively gives an intervention (e.g. vaccine, new drug) and compares outcomes.
  • Randomised controlled trial (RCT) – Participants are randomly assigned to treatment or control groups.

7. Basic Reproduction Number & Herd Immunity

• Basic reproduction number (R₀) – The average number of people one infected person will infect in a completely susceptible population.
• If R₀ > 1 → disease can spread in the population.
  If R₀ < 1 → disease spread will eventually decrease and die out.
• Herd immunity – Indirect protection from an infectious disease that happens when a sufficiently large proportion of the population is immune (through vaccination or previous infection).

Key idea: Higher R₀ → higher percentage of people need to be immune to achieve herd immunity.

8. Prevention, Control & Surveillance

• Primary prevention – Prevent disease before it occurs (e.g. vaccination, health education, sanitation).
• Secondary prevention – Detect disease early and treat it (e.g. screening programmes).
• Tertiary prevention – Reduce the impact of established disease (e.g. rehabilitation, long-term treatment).
• Surveillance – Ongoing, systematic collection, analysis, and interpretation of health data for planning and decision-making.

9. Quick Revision Checklist

1. Define epidemiology and explain why it is important.
2. Explain the difference between incidence, prevalence, morbidity, and mortality.
3. Describe and give examples of endemic, epidemic, and pandemic patterns.
4. Identify the main modes of transmission for communicable diseases.
5. Explain how vectors and fomites contribute to disease spread.
6. Compare cohort and case-control studies.
7. Explain R₀ and how herd immunity helps control disease.
8. Give examples of primary, secondary, and tertiary prevention.
9. Describe the purpose of disease surveillance.