Study for the altarpiece of Saint Rosalie among the Plague-Stricken (ca. 1657–60) by Carlo Maratti (Italian, 1625-1713)

In the realm of epidemiology, the terms epidemic, pandemic, and endemic are fundamental in describing the prevalence and geographic spread of diseases. These terms, often used in public health discussions, especially in the context of outbreaks like COVID-19, help professionals and the public understand the scale and scope of health threats. This Epi Explained delves into the definitions, distinctions, and examples of each, providing a comprehensive understanding for beginners.

Epidemic: A Localized Surge

Definition and Key Features

An epidemic refers to a sudden increase in the number of cases of a disease above what is normally expected in a specific area or population. The key characteristics of an epidemic include:

  • Localized Increase: Epidemics are often confined to a single community, city, or region.
  • Sudden Surge: There’s a rapid rise in disease incidence over a short period.
  • Temporal Pattern: Epidemics typically have a defined time frame, starting with a few cases that exponentially grow, peak, and eventually decline.


Some modern examples of epidemics are often those which are also vaccine preventable. For instance, in recent years some small Measles outbreaks have turned into state or multi-state epidemics in the United States. Measles, which was formally eliminated in the United States in 2000, still can be imported and cause sporadic outbreaks, from a couple of people to several hundred in some communities. It is important to also note that Epidemic does not necessarily equate to any level of severity or lethality; The term Epidemic can be from a disease causing 0 fatalities, or be highly fatal such as Smallpox or Ebola epidemics.

Pandemic: A Global Outbreak

Definition and Key Features

A pandemic is an epidemic that has spread over several countries or continents, usually affecting a large number of people. The defining aspects of a pandemic include:

  • Global Spread: Pandemics cross international boundaries, affecting people worldwide.
  • Massive Scale: The number of individuals impacted is significantly higher than in epidemics.
  • Duration: Pandemics often last longer than epidemics due to their wide geographical spread.


The COVID-19 outbreak, declared a pandemic by the World Health Organization (WHO) in March 2020, is the most recent example. It’s worth noting that even though there have been Epidemic COVID waves with some semblance of seasonality, there’s been consistent cases since 2020.  The H1N1 (swine flu) in 2009 also reached pandemic status, as has some of the most famous disease outbreaks of all time like the so-called Spanish Flu of the 20th century.

Endemic: A Constant Presence

Definition and Key Features

Endemic diseases are constantly present in a particular geographic area or population. These diseases have predictable rates of incidence and prevalence over time. Characteristics of endemic diseases include:

  • Stable Incidence: The number of cases does not fluctuate significantly over time.
  • Geographic Specificity: Endemics are confined to specific areas due to environmental conditions, population immunity, and other factors.
  • Predictability: Health professionals can often predict the number of cases based on past data.


Malaria is endemic in parts of Africa due to the constant presence of the Anopheles mosquito. Similarly, chickenpox was considered endemic in many countries before the widespread use of the varicella vaccine.

Comparison and Contextual Application

Understanding the difference between these terms is crucial for response work. For instance, an epidemic requires immediate, localized interventions, while a pandemic might necessitate global collaboration and resource allocation. Recognizing a disease as endemic helps in developing long-term strategies, such as vaccination programs, to manage its spread. These should not be seen as fully separated categories however, as a proper intervention plan in public health emergency response divisions or for larger programs, should account for the most likely transition of categories for an outbreak and plan accordingly.

Transition from Pandemic to Endemic

A disease can transition from pandemic to endemic status as it becomes integrated into the regular pattern of diseases within a population. COVID-19 is an example where discussions about its potential shift to endemicity are ongoing, indicating the virus might circulate permanently until sufficient efforts and solutions are put forth for eradication. Historically in policy circles, this has been a point of argument against response measures including social distancing and other practical prevention activities, with the false equivalence of something no longer being a pandemic and it being any less dangerous to a population.


Epidemics, pandemics, and endemic diseases represent different scales and patterns of disease spread. While epidemics denote a sudden increase in disease cases in a specific area, pandemics indicate a global outbreak affecting a substantial portion of the world’s population. Endemic diseases, on the other hand, are a constant presence within a particular region. Understanding these differences enhances our grasp of public health strategies needed to combat infectious diseases, reflecting the importance of epidemiology in safeguarding global health.


Humanities Moment

The featured image for this Epi Explained was Study for the altarpiece of Saint Rosalie among the Plague-Stricken (ca.1657–60) by Carlo Maratti (Italian, 1625-1713).  Carlo Maratta was an Italian painter whose work bridged the Late Baroque and Classical styles, primarily active in Rome where he became the preeminent artist following the death of Bernini in 1680. Maratta’s art was influenced by the classical ideals of Raphael and the Baroque dynamism of color, and he became well-known for both his religious and portrait paintings. He led the most prominent art studio in Rome, contributed significantly to the art community as the director of the Accademia di San Luca, and in his later years, turned to restoration work to cope with a decline in patronage.

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