An epidemic is the spread of a disease that is limited to a community, region, or country. An epidemic is a more localized version of a pandemic, which involves the spread of the disease across national borders.

Experts define an epidemic as the spread of a disease within a localized region or community. Once the outbreak spreads across national borders, people may consider it a pandemic or pandemic threat.

This article explores the definitions of an epidemic and a pandemic, how both epidemics and pandemics start, the differences between an epidemic and a pandemic, and pandemic phases.

infographic showing a bird's eye view of people maintaining social distance from each otherShare on Pinterest
Artur Debat/Getty Images

The baseline, or endemic level, of a particular disease is the amount of that disease that is usually present in a community. The endemic level is not necessarily the desired level of the disease, which may be zero. Rather, it is the observed level of the disease within that particular population.

According to the Centers for Disease Control and Prevention (CDC), an epidemic is an increase in the number of cases of a disease above the level experts would expect to see in a specific population.

An epidemic often begins with a sudden increase in the rate of the disease.

According to the CDC, an epidemic may occur when one of the following instances takes place:

  • there is a higher presence of an infectious agent (bacterial infection or virus) in an area where it already existed
  • the infectious agent spreads to an area where it was not present before
  • people who were not initially at risk of infection begin to fall ill due to it
  • when the infectious agent has an enhanced mode of transmission, meaning that more susceptible people become exposed

The CDC places epidemics into four categories. They are:

  • Common-source epidemics: Where a population or group is all exposed to one infectious agent or toxin from the same source.
  • Propagated epidemic: Where the infectious agent spreads throughout a population via transmission from one person to another.
  • Mixed epidemic: This is a combination of common-source and propagated epidemics where a disease spreads to a population via an infectious agent before a secondary spread occurs from person to person.
  • Other epidemics: Other epidemics are not common-source or propagated from one person to another.

According to the World Health Organization (WHO), a pandemic is the worldwide spread of a new disease.

A pandemic occurs when a disease crosses from nation to nation, as opposed to only existing within one nation.

Pandemics often spread across a large geographic area and can cause significant economic, social, and political disruption.

Often, the disease that causes a pandemic can cause severe illness and spreads quickly from person to person.

The severity of the illness may be higher during a pandemic due to the fact that the people the disease spreads to often have no pre-existing immunity to the new infectious agent.

Pandemics from recent history include:

  • 1918 – 1920: H1N1 virus
  • 1957 – 1958: H2N2 virus
  • 1968–1969: H3N2 virus
  • 2009–2010: Novel Influenza A H1N1, the virus that causes “swine flu
  • 2020: SARS (Severe Acute Respiratory Syndrome)-CoV-2, the virus that causes COVID-19 (coronavirus disease)

Other historical pandemics include The Black Death (1346–1350) and the sixth cholera pandemic (1899–1923).

Animal virus risks

Animals carry viruses that rarely spread to humans. However, if a mutation occurs, these mutations sometimes pass on to the human population.

Health authorities often become very concerned if animal viruses become transmissible to humans.

This is because they often become highly contagious and harmful. Swine flu and COVID-19 are examples of deadly pandemics that began due to the spread of a virus from animals to humans.

The WHO’s six-phase pandemic program

The WHO has outlined a six-phase program for identifying potential influenza pandemics.

These phases are:

  • Phase 1: Health authorities have not reported that a virus circulating among animals can spread to humans.
  • Phase 2: A health authority has reported that an animal influenza virus circulating among animals has caused infection in humans. Once this occurs the WHO considers this a potential threat of a pandemic.
  • Phase 3: An animal or human-animal influenza virus has spread to small clusters of people. However, the virus has not caused rapid human-to-human transmission that is sufficient enough to sustain community-level outbreaks.
  • Phase 4: Human-to-human transmission of an animal or human-animal influenza virus is able to spread quickly enough to sustain a community-level outbreak.
  • Phase 5: The virus causes a sustained community-level outbreak in two or more different nations in one WHO region.
  • Phase 6: In addition to the criteria in Phase 5, the same virus has also spread and caused a community-level outbreak in at least one other country in a different WHO region.

Post-peak period

The WHO also outlines a “post-peak period” when the level of the virus in most countries with adequate surveillance drops to below peak levels.

Post-pandemic period

Finally, the WHO states that the pandemic has entered a “post-pandemic period” when the levels of the virus have returned to the usual levels of seasonal influenza in most countries with adequate surveillance.

An epidemic is a disease that spreads and affects a large number of people within a community, population, or region.

A pandemic is an epidemic that has spread across national borders to different countries and continents of the world.

When a disease suddenly begins to spread it can cause an epidemic or a pandemic.

An epidemic is the spread of a disease throughout a region or community that does not cross national borders.

During an epidemic, the number of cases of a disease must rise above the normal expected amount in a specific population.

A pandemic is similar to an epidemic but involves the spread of the disease across national borders.

A pandemic often spreads across a large area, affecting many nations. Pandemics can cause significant economic, social, and political disruption.