Endemic, epidemic, pandemic: What's the difference?

Published 7 June 2026 · health

The Andes hantavirus outbreak shows how easily disease labels are misunderstood — and why they describe patterns of spread, not how dangerous a virus is

The Andes hantavirus outbreak shows how easily disease labels are misunderstood — and why they describe patterns of spread, not how dangerous a virus is. The recent outbreak of the rare Andes hantavirus has once again drawn attention to terms that became widely familiar during the COVID‑19 pandemic: endemic, epidemic and pandemic. These concepts are often grouped together or misused in public discourse, but in epidemiology they have precise meanings. Importantly, they describe how a disease spreads — not how dangerous it is. So what do the terms actually mean? Endemic: A constant threat A disease that occurs regularly in certain regions is called endemic. When a disease becomes endemic, the number of people falling ill remains relatively constant over time. The number of cases is higher than in other areas but does not increase over time. Over a certain period of time, approximately the same number of people repeatedly contract the disease. A typical example is malaria, which annually affects 300 million people worldwide, with most cases in the tropics. Endemic does not mean harmless. Endemic diseases can be severe or fatal. The defining feature is not their impact on individuals but their steady, geographically limited occurrence.

Epidemic: Only in one region The spread of a disease is termed an epidemic when it occurs with unusual frequency in a certain region and for a limited period of time. When the number of cases of a disease in that particular region rises above the expected (endemic) level, it is called an epidemic. When the disease incidence is localized, it's often referred to as an outbreak. An epidemic occurs, for example, when the virulence of a particular pathogen changes: a virus mutates and becomes more contagious. Epidemics can also occur when diseases are newly introduced into a certain area. The prerequisite is that a disease can be passed from person to person. An early example of this is smallpox, which was introduced to the Americas via the arrival of Europeans from the beginning of the 16th century. Because the Indigenous population had never before been in contact with the pathogens, their immune systems could not fend off the virus. Individual projections suggest that up to 90% of the Indigenous population of the Americas fell victim to smallpox. Pandemic: Worldwide spread If a disease spreads across countries and continents, experts refer to it as a pandemic.

This means, above all, that successful control of the disease depends on cooperation among the health systems of different countries. It does not mean that a disease is particularly dangerous or deadly. According to the WHO and the US Centers for Disease Control and Prevention (CDC), pandemics are usually caused by newly emerging pathogens or virus types. For example, these can be zoonoses — diseases that are transmitted from animals to humans. If a disease is new to humans, very few people will be immune to the virus. Vaccinations are also not available in this case. This can lead to vast numbers of people becoming infected. How dangerous or fatal the disease is depends on the specific virus and the health of the individual. Even if, in percentage terms, a disease is harmless in most cases, the absolute number of serious illnesses during a pandemic can be very high. This is simply because a very large number of people are infected with the pathogens overall. A typical disease that repeatedly assumes pandemic proportions is influenza. The 1918 influenza pandemic, also known as the Spanish flu, killed 25 million to 50 million people — that's more people than the victims of World War I.