A pandemic is when a new disease or new strain of an existing disease spreads worldwide. An outbreak of infection is considered a pandemic when it grows very fast. It starts out slowly at first and then takes off. Sometimes, people call this type of growth exponential. For an outbreak to be a pandemic, it also has to affect lots of people in many countries or even continents. It spreads quickly and easily around the world all at once.

Viral respiratory diseases, meaning viruses that move through the air and are breathed in, are the ones most likely to cause a pandemic. Pandemics usually happen when a virus such as the coronavirus that causes COVID-19 starts infecting people for the first time. New viruses can spread and infect you more easily than those that have been around before. That's because you don't have any immunity against them. You usually won't be able to get a vaccine or other medicines to protect you against a new virus either.

At first, doctors might not even know the identity of the new virus, how it spreads, or the best way to slow it down. Any type of virus may cause a pandemic, but some are more likely to do so than others. While the COVID-19 pandemic was caused by a coronavirus, many other pandemics in the past were caused by new strains of influenza or other viruses.

In a pandemic, new viruses infect lots of people and make them sick. But how sick you or others will get depends on the virus. It also will depend on other health conditions you have and how the new virus interacts with those. So, some pandemics are deadlier than others. The key factor in a pandemic is that an outbreak of infection is both fast-moving and widespread.

Throughout history, pandemics of diseases such as influenza, cholera, and plague have played a major role in shaping human civilizations. Examples of significant historical pandemics include the plague pandemic of the Byzantine Empire in the 6th century; the Black Death, which originated in China and spread across Europe in the 14th century; and the Great Influenza pandemic of 1918-1919, which originated in the United States, in Kansas and spread to Europe, Asia, and islands in the South Pacific. Although pandemics typically occur over a short span of time, today several infectious diseases persist at a high level of incidence, occur on a global scale, and can be transmitted between humans either directly or indirectly. Such diseases represented in modern pandemics include AIDS, caused by HIV (human immunodeficiency virus), which is transmitted directly between humans; and malaria, caused by parasites in the genus Plasmodium, which are transmitted from one human to another by mosquitoes that feed on the blood of infected humans (Rogers, 2025).

Who is at risk?

Zoonotic pathogens can spread to humans through any contact point with domestic, agricultural or wild animals. Markets selling the meat or other parts of wild animals are particularly high risk due to the large number of new pathogens known to exist in some wild animal populations. Agricultural workers in areas with a high use of antibiotics for farm animals may be at increased risk of pathogens resistant to current antimicrobial drugs. People living near wilderness areas or in semi-urban areas with higher numbers of wild animals are at risk of disease from animals such as rats, foxes or raccoons. Urbanization and the destruction of natural habitats increase the risk of zoonotic diseases by increasing contact between humans and wild animals.