Levels Of A Disease: Endemic vs Epidemic vs Pandemic

The amount of a particular disease that is usually present in a community is referred to as the baseline or endemic level of the disease.

This level is not necessarily the desired level, which may in fact be zero, but rather is the observed level.

In the absence of intervention and assuming that the level is not high enough to deplete the pool of susceptible persons, the disease may continue to occur at this level indefinitely.

Thus, the baseline level is often regarded as the expected level of the disease.

While some diseases are so rare in a given population that a single case warrants an epidemiologic investigation (e.g., rabies, plague, polio), other diseases occur more commonly so that only deviations from the norm warrant investigation.

Sporadic refers to a disease that occurs infrequently and irregularly. Endemic refers to the constant presence and/or usual prevalence of a disease or infectious agent in a population within a geographic area. Hyperendemic refers to persistent, high levels of disease occurrence.

RELATED: Coronavirus Health Level Raised To Pandemic

Occasionally, the amount of disease in a community rises above the expected level. Epidemic refers to an increase, often sudden, in the number of cases of a disease above what is normally expected in that population in that area.

Outbreak carries the same definition of epidemic, but is often used for a more limited geographic area.

Cluster refers to an aggregation of cases grouped in place and time that are suspected to be greater than the number expected, even though the expected number may not be known.

Pandemic refers to an epidemic that has spread over several countries or continents, usually affecting a large number of people.

Epidemics occur when an agent and susceptible hosts are present in adequate numbers, and the agent can be effectively conveyed from a source to the susceptible hosts.

More specifically, an epidemic may result from:

  • A recent increase in amount or virulence of the agent,
  • The recent introduction of the agent into a setting where it has not been before,
  • An enhanced mode of transmission so that more susceptible persons are exposed,
  • A change in the susceptibility of the host response to the agent, and/or
  • Factors that increase host exposure or involve introduction through new portals of entry

A History of Pandemic

HIV/AIDS PANDEMIC (AT ITS PEAK, 2005-2012)
Death Toll: 36 million
Cause: HIV/AIDS

First identified in Democratic Republic of the Congo in 1976, HIV/AIDS has truly proven itself as a global pandemic, killing more than 36 million people since 1981.

Currently there are between 31 and 35 million people living with HIV, the vast majority of those are in Sub-Saharan Africa, where 5% of the population is infected, roughly 21 million people.

As awareness has grown, new treatments have been developed that make HIV far more manageable, and many of those infected go on to lead productive lives.

Between 2005 and 2012 the annual global deaths from HIV/AIDS dropped from 2.2 million to 1.6 million.

FLU PANDEMIC (1968)
Death Toll: 1 million
Cause: Influenza

A category 2 Flu pandemic sometimes referred to as “the Hong Kong Flu,” the 1968 flu pandemic was caused by the H3N2 strain of the Influenza A virus, a genetic offshoot of the H2N2 subtype.

From the first reported case on July 13, 1968 in Hong Kong, it took only 17 days before outbreaks of the virus were reported in Singapore and Vietnam, and within three months had spread to The Philippines, India, Australia, Europe, and the United States.

While the 1968 pandemic had a comparatively low mortality rate (.5%) it still resulted in the deaths of more than a million people, including 500,000 residents of Hong Kong, approximately 15% of its population at the time.

ASIAN FLU (1956-1958)
Death Toll: 2 million
Cause: Influenza

Asian Flu was a pandemic outbreak of Influenza A of the H2N2 subtype, that originated in China in 1956 and lasted until 1958. In its two-year spree, Asian Flu traveled from the Chinese province of Guizhou to Singapore, Hong Kong, and the United States.

Estimates for the death toll of the Asian Flu vary depending on the source, but the World Health Organization places the final tally at approximately 2 million deaths, 69,800 of those in the US alone.

FLU PANDEMIC (1918)
Death Toll: 20 -50 million
Cause: Influenza

Between 1918 and 1920 a disturbingly deadly outbreak of influenza tore across the globe, infecting over a third of the world’s population and ending the lives of 20 – 50 million people.

Of the 500 million people infected in the 1918 pandemic, the mortality rate was estimated at 10% to 20%, with up to 25 million deaths in the first 25 weeks alone.

What separated the 1918 flu pandemic from other influenza outbreaks was the victims; where influenza had always previously only killed juveniles and the elderly or already weakened patients, it had begun striking down hardy and completely healthy young adults, while leaving children and those with weaker immune systems still alive.

SIXTH CHOLERA PANDEMIC (1910-1911)
Death Toll: 800,000+
Cause: Cholera

Like its five previous incarnations, the Sixth Cholera Pandemic originated in India where it killed over 800,000, before spreading to the Middle East, North Africa, Eastern Europe and Russia. The Sixth Cholera Pandemic was also the source of the last American outbreak of Cholera (1910–1911).

American health authorities, having learned from the past, quickly sought to isolate the infected, and in the end only 11 deaths occurred in the U.S. By 1923 Cholera cases had been cut down dramatically, although it was still a constant in India.

FLU PANDEMIC (1889-1890)
Death Toll: 1 million
Cause: Influenza

Originally the “Asiatic Flu” or “Russian Flu” as it was called, this strain was thought to be an outbreak of the Influenza A virus subtype H2N2, though recent discoveries have instead found the cause to be the Influenza A virus subtype H3N8.

The first cases were observed in May 1889 in three separate and distant locations, Bukhara in Central Asia (Turkestan), Athabasca in northwestern Canada, and Greenland. Rapid population growth of the 19th century, specifically in urban areas, only helped the flu spread, and before long the outbreak had spread across the globe.

Though it was the first true epidemic in the era of bacteriology and much was learned from it. In the end, the 1889-1890 Flu Pandemic claimed the lives of over a million individuals.

THIRD CHOLERA PANDEMIC (1852–1860)
Death Toll: 1 million
Cause: Cholera

Generally considered the most deadly of the seven cholera pandemics, the third major outbreak of Cholera in the 19th century lasted from 1852 to 1860.

Like the first and second pandemics, the Third Cholera Pandemic originated in India, spreading from the Ganges River Delta before tearing through Asia, Europe, North America and Africa and ending the lives of over a million people.

British physician John Snow, while working in a poor area of London, tracked cases of cholera and eventually succeeded in identifying contaminated water as the means of transmission for the disease. Unfortunately the same year as his discovery (1854) went down as the worst year of the pandemic, in which 23,000 people died in Great Britain.

THE BLACK DEATH (1346-1353)
Death Toll: 75 – 200 million
Cause: Bubonic Plague

From 1346 to 1353 an outbreak of the Plague ravaged Europe, Africa, and Asia, with an estimated death toll between 75 and 200 million people. Thought to have originated in Asia, the Plague most likely jumped continents via the fleas living on the rats that so frequently lived aboard merchant ships.

Ports being major urban centers at the time, were the perfect breeding ground for the rats and fleas, and thus the insidious bacterium flourished, devastating three continents in its wake.

PLAGUE OF JUSTINIAN (541-542)
Death Toll: 25 million
Cause: Bubonic Plague

Thought to have killed perhaps half the population of Europe, the Plague of Justinian was an outbreak of the bubonic plague that afflicted the Byzantine Empire and Mediterranean port cities, killing up to 25 million people in its year long reign of terror.

Generally regarded as the first recorded incident of the Bubonic Plague, the Plague of Justinian left its mark on the world, killing up to a quarter of the population of the Eastern Mediterranean and devastating the city of Constantinople, where at its height it was killing an estimated 5,000 people per day and eventually resulting in the deaths of 40% of the city’s population.

ANTONINE PLAGUE (165 AD)
Death Toll: 5 million
Cause: Unknown


Also known as the Plague of Galen, the Antonine Plague was an ancient pandemic that affected Asia Minor, Egypt, Greece, and Italy and is thought to have been either Smallpox or Measles, though the true cause is still unknown.

This unknown disease was brought back to Rome by soldiers returning from Mesopotamia around 165AD; unknowingly, they had spread a disease which would end up killing over 5 million people and decimating the Roman army.