Epidemic

An epidemic refers to the rapid spread of a disease within a population, leading to a level of incidence significantly above the normal expectation for a given area and period. Derived from the Ancient Greek epi (“upon”) and demos (“people”), the concept captures sudden increases in disease prevalence that may range from localised outbreaks to widespread regional events. While some epidemics affect a single community, others may extend across countries or continents; when this occurs on a large scale, the phenomenon may be classified as a pandemic. Epidemics can impose profound health, social, and economic impacts, disrupting societies and challenging public health systems.
The United States Centers for Disease Control and Prevention defines an epidemic broadly as any increase, often sudden, in the number of cases above what is normally expected for a particular population. By contrast, the term outbreak is generally reserved for smaller, localised episodes. Whether an observed rise in disease constitutes an epidemic depends on a clear understanding of baseline incidence. Consequently, a few cases of a very rare disease may be considered epidemic, whereas elevated numbers of a common illness such as the cold may not qualify.

Historical Understanding and Origins of the Term

Historical accounts of epidemics reveal early attempts to describe and understand unusual disease clusters. One of the most famous early descriptions is Thucydides’ account of the Plague of Athens during the Peloponnesian War, which remains a foundational text in the study of population-level disease outbreaks. The term epidemic appears in the works of Hippocrates, particularly in Of the Epidemics, where it began to acquire its medical meaning.
By the early seventeenth century, epidemic and endemic became commonly recognised contrasting concepts. An endemic condition referred to a persistent, commonly occurring sickness in a region, whereas an epidemic denoted a health event appearing at a specific time and affecting many people simultaneously. Although epizootic is the precise equivalent term for disease outbreaks in non-human animals, the word epidemic is sometimes applied more broadly in scientific and popular contexts.

Causes of Epidemics

Epidemics arise from complex interactions among pathogens, hosts, and environmental conditions. Several factors may contribute individually or in combination to triggering a rapid increase in disease incidence.

Antigenic Change

Antigenic variation plays a crucial role in the emergence and spread of infectious diseases. Antigens are proteins on a pathogen’s surface that provoke an immune response. When antigenic structures change, existing immunity may no longer provide protection, allowing the pathogen to spread more easily through a population. Two natural mechanisms drive antigenic change:

• Antigenic drift, involving gradual accumulation of mutations in viral genomes, producing strains that can evade host immunity.
• Antigenic shift, an abrupt process in which two or more viral strains co-infect a host and combine to form a novel subtype with distinct antigenic properties.

Influenza viruses provide the most well-documented examples of both processes, and similar antigenic evolution has been observed in pathogens such as SARS-CoV-2.

Drug Resistance

Antimicrobial resistance is a further contributor to epidemic emergence, as resistant pathogens are more difficult to treat and control. Bacteria may acquire resistance through spontaneous mutation or horizontal gene transfer. Prolonged or inappropriate antibiotic use accelerates the selection of resistant strains. Multidrug-resistant tuberculosis is a notable example, posing a persistent public health challenge in many parts of the world.

Changes in Transmission Dynamics

Pathogens spread through a variety of transmission modes, including airborne, waterborne, vector-borne, and sexually transmitted pathways. Alterations in transmission conditions can significantly influence epidemic behaviour. Some pathogens may develop increased survival outside the host, enhancing transmissibility and virulence, while others may broaden their transmission routes, although such adaptations are rare. Human activities—such as poor sanitation, increased travel, or ecological disruption—can also facilitate more efficient dissemination.

Seasonality

Many diseases display seasonal patterns influenced by environmental and behavioural factors. These may include:

• environmental conditions that affect pathogen survival, such as humidity and temperature
• behavioural changes, including increased indoor crowding during colder seasons
• immune system fluctuations, potentially linked to reduced sunlight exposure and vitamin D
• variations in vector abundance, especially in the case of mosquito-borne diseases

Examples include winter peaks of influenza in temperate regions and cholera outbreaks associated with monsoonal rainfall.

Human Behaviour

Human behaviour significantly shapes epidemic outcomes. The 1854 Broad Street cholera outbreak, mitigated by removal of a contaminated water pump handle, exemplifies how behaviour-linked interventions can halt the spread of disease. Urbanisation, overcrowding, inadequate sanitation, and cultural practices such as traditional funeral rites can accelerate transmission. The spread of Ebola virus disease in West Africa was exacerbated by rituals involving close contact with the bodies of the deceased, prompting behavioural change campaigns as part of control measures.

Changes in Host Population Immunity

Population immunity fluctuates over time. Following an outbreak or vaccination programme, herd immunity is high, but wanes as individuals lose immunity or as susceptible individuals are born. Without continued vaccination or exposure, populations become vulnerable to new epidemics. Diseases endemic in one region may trigger epidemics when introduced into populations without prior exposure, as seen when European diseases were transmitted to Indigenous peoples in the sixteenth century.

Zoonosis

Zoonoses are diseases that originate in non-human animals and cross the species barrier to infect humans. Many major diseases, including Ebola virus disease and salmonellosis, have zoonotic origins. HIV is believed to have crossed into humans in the early twentieth century, later evolving into a distinct human-adapted virus. Influenza A viruses, such as H5N1 and H1N1, are well-known zoonoses capable of recombining with human strains, occasionally producing global pandemics.

Types of Epidemics

Epidemics can be categorised based on how the disease is transmitted within a population.

Common Source Outbreaks

In a common source outbreak, all affected individuals are exposed to the same infectious agent or toxin. These outbreaks may take several forms:

• Point source outbreaks, where exposure occurs at a single point in time and cases develop within one incubation period.
• Continuous outbreaks, where exposure occurs over an extended period.
• Intermittent outbreaks, in which exposure varies over time.

Such outbreaks are often associated with contaminated water, food, or environmental sources.

Propagated Outbreaks

Propagated outbreaks occur through person-to-person transmission. In these cases, each infected individual may become a source of further infection, allowing cases to extend across multiple generations of transmission. This pattern is typical of diseases such as influenza, measles, or COVID-19.