Epidemiology Vocabulary in English

An epidemic is a sudden increase in disease cases above what is normally expected within a specific area or population. An pandemic is an epidemic that has spread across multiple countries or continents and is affecting a large proportion of the global population. The distinction is primarily geographic: an epidemic is contained to a region, while a pandemic crosses international borders on a wide scale. The term endemic refers to a disease that is consistently present at relatively stable levels in a particular area without external causes — for example, malaria in parts of Africa or seasonal flu in temperate countries. The WHO has the authority to declare a pandemic, which triggers coordinated international responses.

The R number (or reproduction number, technically R0 for the basic reproduction number) represents the average number of people that one infected person will go on to infect in a fully susceptible population. An R number above 1 means each infected person infects more than one other person on average, so the outbreak is growing exponentially. An R number below 1 means the outbreak is shrinking because infected people are infecting fewer than one additional person on average. An R number of exactly 1 means the outbreak is stable. The R number is not a fixed property of a disease; it depends on the population’s behaviour, immunity levels, and the measures in place. During the COVID-19 pandemic, the R number became a central tool for communicating the status of the outbreak to the public.

Incidence measures the rate at which new cases of a disease arise in a population over a specific time period. It is a measure of risk: how likely is someone to develop the disease? Prevalence measures the total proportion of a population that currently has the disease at a given point in time (point prevalence) or over a period (period prevalence). It includes both new and existing cases. For acute diseases that resolve quickly, incidence and prevalence tend to be similar. For chronic diseases that last a long time — such as diabetes or HIV — prevalence can be much higher than incidence, because cases accumulate over time. Epidemiologists use both measures to understand disease burden and plan health services.

Quarantine applies to people who may have been exposed to a contagious disease but are not yet known to be infected. The purpose is to restrict their movement during the incubation period to prevent them from infecting others if they develop the disease. Isolation applies to people who are confirmed to be infected. The purpose is to separate them from healthy people to prevent transmission. During the COVID-19 pandemic, both terms were widely used but often confused. Quarantine typically lasts for the duration of the incubation period of the disease in question; isolation lasts until the person is no longer considered infectious, which may be determined by time elapsed or by a negative test.

Herd immunity is the indirect protection that occurs when a sufficiently large proportion of a population has become immune to an infectious disease — whether through vaccination or prior infection — so that the disease can no longer spread easily even among those who are not immune. The proportion of the population that needs to be immune to achieve herd immunity varies by disease and depends on the R0: for measles, with an R0 of 12–18, approximately 95% of the population needs to be immune. For COVID-19, estimates varied but were typically between 60% and 90%. Herd immunity is the basis for national vaccination programmes and the reason that high vaccination coverage protects vulnerable individuals who cannot be vaccinated for medical reasons.

Contact tracing is the process of identifying people who have been in close contact with an infected person and notifying them so they can be tested, monitored, or quarantined. It is one of the oldest and most effective public health tools for controlling infectious disease outbreaks. When a new case is identified, contact tracers interview the person to establish who they were in close contact with during their infectious period. Those contacts are then notified, advised on symptoms to watch for, and, where appropriate, asked to quarantine. Digital contact tracing apps, which use Bluetooth signals to detect proximity, were developed during COVID-19 as a way of scaling up tracing capacity. The effectiveness of contact tracing depends on speed: the sooner contacts are reached, the less opportunity there is for further transmission.

The case fatality rate (CFR) is the proportion of people confirmed to have a disease who die from it. It measures the severity of a disease among those who are known to be infected. The mortality rate (more precisely, the infection fatality rate or crude death rate) is the proportion of the overall population, or of all infected individuals including undetected cases, who die. During an outbreak, the CFR can be misleading: if only the sickest cases are tested and confirmed, the CFR will appear very high. If testing is widespread and mild cases are detected, the CFR falls. This is why seroprevalence studies (measuring antibodies in the population) are important — they reveal the true number of infections and allow a more accurate infection fatality rate to be calculated.

Confounding occurs when the apparent relationship between an exposure and an outcome is distorted by a third variable — the confounder — that is associated with both. For example, studies once found that coffee drinking was associated with lung cancer. However, this was because coffee drinkers were also more likely to smoke, and smoking was the real cause of the lung cancer. Smoking was the confounder. Epidemiologists use various methods to control for confounding, including randomisation (in clinical trials), restriction, matching, and statistical adjustment. Recognising and controlling for confounding is one of the central challenges of observational epidemiology and is why randomised controlled trials are considered the gold standard of evidence.

Surveillance in epidemiology is the continuous, systematic collection, analysis, and interpretation of health data, used to monitor disease trends and detect outbreaks early. Surveillance systems collect data from sources including hospital admissions, laboratory tests, death certificates, GP consultations, and pharmacy sales. This data is analysed to identify unusual patterns that might signal an emerging outbreak. For example, a sudden spike in sales of cough medicines, or an unusual cluster of pneumonia cases in a particular hospital, might trigger an investigation. The WHO coordinates global disease surveillance through systems such as the Global Outbreak Alert and Response Network (GOARN), allowing rapid sharing of information about emerging threats across borders.

Group the words into conceptual clusters: disease occurrence (epidemic, pandemic, endemic, incidence, prevalence), transmission and spread (R number, transmission, vector, incubation period, herd immunity), control measures (quarantine, isolation, contact tracing, surveillance), outcomes (mortality rate, case fatality rate, seroprevalence), and research methods (cohort study, risk factor, confounding). Use Flash Cards to build accurate recall of definitions, then read public health reports from the WHO, NHS England, or Public Health England. Reading coverage of the COVID-19 pandemic from quality news sources is particularly valuable, as it offers examples of most of these words used in real-world contexts that many learners already have background knowledge about.