Public Health Vocabulary in English

An epidemic is a sudden increase in the number of cases of a disease above what is normally expected, within a specific community or region. A pandemic is an epidemic that has spread across multiple countries or continents, affecting a large number of people worldwide. The World Health Organisation (WHO) officially declares a pandemic. The COVID-19 outbreak became a pandemic in March 2020.

Incidence refers to the number or rate of new cases of a disease occurring in a population over a defined period — it measures how quickly a disease is developing. Prevalence refers to the total number or proportion of cases (both old and new) existing in a population at a specific point in time. Incidence is useful for understanding disease spread; prevalence is useful for understanding the disease burden on health services.

Social determinants of health are the non-medical factors that influence health outcomes — the conditions in which people are born, grow, live, work, and age. They include income and social status, education, employment, housing, access to healthcare, and social support networks. Research consistently shows that people living in poverty or in deprived areas have significantly worse health outcomes than those in more affluent circumstances.

Herd immunity (also called population immunity) occurs when enough people in a population are immune to a disease — through vaccination or prior infection — that the disease can no longer spread efficiently, protecting even those who are not immune. The threshold for herd immunity varies by disease: measles requires around 95% immunity; COVID-19 estimates varied. Vaccination is the safest route to achieving herd immunity without widespread illness.

A disease is endemic when it is consistently present in a particular region or population at a relatively stable and expected level — for example, malaria in parts of sub-Saharan Africa. An epidemic is when cases increase suddenly and significantly above the expected level. A disease can transition between endemic and epidemic states: if an endemic disease spikes, it becomes an epidemic.

Contact tracing is the process of identifying, informing, and monitoring people who may have been exposed to an infectious disease through contact with a confirmed case. It is a core tool in controlling infectious disease outbreaks. Contact tracers interview infected individuals to identify their recent contacts, then notify those contacts so they can be tested or quarantined. Digital contact tracing apps were widely used during the COVID-19 pandemic.

Disease surveillance is the continuous, systematic collection, analysis, and interpretation of health-related data. It allows public health authorities to detect outbreaks early, monitor trends, and evaluate the effectiveness of interventions. Surveillance data can come from hospitals, laboratories, GPs, death registrations, or community surveys. In the UK, Public Health England (now UKHSA) operates national disease surveillance systems.

Morbidity refers to the state of being diseased or the rate of disease or illness within a population. Mortality refers to death or the death rate. A disease can have high morbidity (many people become ill) but low mortality (few die), such as the common cold. Conversely, some conditions have low morbidity but high mortality. Both metrics are used to measure the impact of disease on populations.

A zoonosis (plural: zoonoses) is an infectious disease that can be transmitted from animals to humans, or from humans to animals. Examples include rabies, avian influenza, Ebola, salmonella, and COVID-19 (which is believed to have originated from an animal source). Around 60% of known infectious diseases in humans are zoonotic. Monitoring animal populations is therefore an important part of public health surveillance.

Health promotion is the process of enabling people to increase control over their own health and improve it. It goes beyond treating illness to address the root causes of poor health through education, policy, community action, and environmental changes. Examples include anti-smoking campaigns, healthy eating initiatives, and mental health awareness programmes. The Ottawa Charter (1986) is a foundational document in health promotion.