Epidemiology: Outbreaks, Epidemics, and Pandemics

Epidemiology is the study of how diseases are distributed in populations and what causes them, and it classifies disease occurrence as endemic (constantly present), an outbreak (a sudden increase in a place), an epidemic (a widespread increase in a region) or a pandemic (an epidemic spreading across several countries or continents). In brief: epidemiologists track cases, calculate rates, find the source and route of spread and recommend prevention. A key number is R₀, the basic reproduction number: the average number of people that one infected person infects in a fully susceptible population. If R₀ is above 1, the disease spreads.

Epidemiology guides vaccination programs, hygiene campaigns and responses to outbreaks, from cholera to COVID-19. This guide explains the key terms, how outbreaks are investigated, how R₀ and herd immunity work, the main prevention methods, works through a numerical example on spread and vaccination coverage and corrects common mistakes.

What are the key terms in epidemiology?

• Incidence: the number of new cases in a population during a period of time, often per 100 000 people.
• Prevalence: the total number of cases (new and existing) at a given time, or the proportion of the population affected.
• Mortality rate: the number of deaths from a disease in a population over a period.
• Case fatality rate (CFR): the proportion of people with a disease who die from it. It can differ from the infection fatality rate (IFR), which includes mild or undetected infections.
• Endemic: a disease that is always present at a fairly constant level in an area, such as malaria in parts of Africa.
• Outbreak: a sudden increase in cases above what is expected, in a limited place or group, such as food poisoning at a wedding.
• Epidemic: an outbreak that spreads to many people over a larger area, such as the 2014 to 2016 Ebola epidemic in West Africa.
• Pandemic: an epidemic that spreads across multiple countries or continents, affecting a large number of people. Examples are the 1918 influenza, HIV/AIDS and COVID-19. The World Health Organization declared COVID-19 a pandemic on 11 March 2020.

Outbreak, epidemic and pandemic compared

How is an outbreak investigated?

1. Confirm the outbreak by checking that cases are above the expected number and that the diagnosis is correct.
2. Define and count cases: who is ill, where, and since when.
3. Describe the pattern: plot an epidemic curve (cases per day), a map of the cases and a list of ages and other characteristics of those affected. The shape of the epidemic curve suggests whether the source is a single event (a sharp peak) or person-to-person spread (successive waves).
4. Form a hypothesis about the source, such as a contaminated food or water supply, and test it by interviewing people and comparing cases with healthy people.
5. Test samples from patients, food and the environment in a laboratory.
6. Control and prevent: remove the source, treat patients, isolate cases, trace contacts and vaccinate.
7. Communicate findings to the public and health authorities.

The classic example is John Snow’s 1854 map of cholera deaths in London, which showed a cluster around the Broad Street water pump.

What is R₀, and what is herd immunity?

The basic reproduction number R₀ depends on how infectious the pathogen is, how long a person is infectious and how many contacts they have. Estimates for different diseases are approximate and vary between studies and places: seasonal influenza about 1.3, the original strain of SARS-CoV-2 about 2 to 3 and measles about 12 to 18.

• If R₀ > 1, each case infects more than one other person on average, and the epidemic grows.
• If R₀ = 1, the disease is stable (endemic).
• If R₀ < 1, the epidemic dies out.

The effective reproduction number, R (or Rₜ), applies at a given time, taking into account immunity, vaccination and control measures. Herd immunity occurs when enough people are immune so that the chain of transmission is broken and the disease cannot spread easily, which also protects people who are not immune. The approximate threshold for the proportion that must be immune is 1 − 1/R₀.

Worked example: spread and vaccination threshold

Part 1: spread without control

A new disease has R₀ = 3. One infected person starts an outbreak. Assume that each generation of infection takes one week and that everyone is susceptible.

1. Generation 0: 1 case.
2. Generation 1: 1 × 3 = 3 cases.
3. Generation 2: 3 × 3 = 9 cases.
4. Generation 3: 27 cases. Generation 4: 81 cases. Generation 5: 243 cases.

The number of new cases is 3ⁿ in generation n. After 10 weeks (generation 10), there would be 3¹⁰ = 59 049 new cases in that week if nothing limited the spread. In reality, spread slows as people become immune or behavior changes.

Part 2: herd immunity threshold

1. For R₀ = 3: threshold = 1 − 1/3 = 2/3 ≈ 67 percent.
2. For measles with R₀ ≈ 15: threshold = 1 − 1/15 = 14/15 ≈ 93 percent. This is why measles outbreaks occur when vaccination coverage falls below about 95 percent.
3. If the vaccine is only 90 percent effective, the coverage needed rises: the required coverage = threshold ÷ effectiveness. For R₀ = 3: 0.667 ÷ 0.90 = 0.74, so 74 percent of the population should be vaccinated.

These are idealized estimates that assume that people mix at random. In real populations, groups with low coverage can still have outbreaks even if the national average is high.

How is disease prevented and controlled?

• Vaccination: the most effective way to prevent many infectious diseases.
• Surveillance: continuous collection of data on diseases, so outbreaks are detected early.
• Hygiene and sanitation: clean water, safe food, sewage treatment, hand-washing.
• Isolation and quarantine; contact tracing: keep those who are infected or exposed from spreading the disease.
• Vector control, travel measures and public information.
• Treatment reduces the duration of infectiousness.
• For non-infectious diseases (such as heart disease and cancer), prevention includes diet, exercise and avoiding smoking and excess alcohol, which are the subject of the articles on human health.

How did epidemiology develop?

The Black Death (1347 to 1351) killed an estimated one third or more of Europe’s population, and led to the first quarantine measures. John Snow is often called the father of modern epidemiology. The 1918 to 1919 influenza pandemic is estimated to have infected about a third of the world’s population and killed at least 50 million people. Smallpox was declared eradicated in 1980 following a global vaccination program. Mathematical models of epidemics, such as the SIR model of Kermack and McKendrick in 1927, are now used routinely.

Which mistakes are common?

Mistake 1: Using epidemic and pandemic as the same term

A pandemic is spread across several countries or continents. An epidemic is more limited in area.

Mistake 2: Saying that a high number of cases means a high death rate

Case numbers measure spread. Death rates measure severity. A disease can spread widely with few deaths.

Mistake 3: Confusing incidence and prevalence

Incidence counts new cases. Prevalence counts all cases at a time.

Mistake 4: Thinking that herd immunity means that no one gets infected

It means that the spread is limited. Some individual cases can still occur.