Special Report

Worst Outbreaks of All Time

Detailed Findings

The Centers for Disease Control and Prevention defines an epidemic as “an increase, often sudden, in the number of cases of a disease above what is normally expected in that population in that area.” The CDC also explains that though “outbreak” has the same definitions as “epidemic,” “outbreak” is used often for in a limited geographic zone.

Pandemic describes an epidemic that has spread to several countries and affects many people. The most recent flu pandemic, H1N1 in 2009, claimed relatively fewer victims especially among the elderly. The 1968-69 Hong Kong Flu killed about 1 million people in Asia, Australia, Europe, and the United States. Many of those who died were 65 years and older.

J.N. Hays, history professor emeritus at Loyola University in Chicago, has authored several books on the impact diseases have had on humankind, such as “Epidemics and Pandemics: Their Impacts on Human History” and “The Burdens of Disease: Epidemics and Human Response in Western History.”

Hays says that while it is true epidemics have shaped societies, societies have shaped epidemics. “Conditions in society predispose the path of epidemics. Who is affected most by epidemics? The poor.”

Whether it is an epidemic or a pandemic, the results can be history-changing. Among the worst pandemics are plagues, life-threatening infectious diseases spread by different animals to humans by fleas. The worst two forms of plague are bubonic and pneumonic, according to the World Health Organization..

Perhaps the most famous pandemic was the bubonic plague, or Black Death, in the 14th century. That pandemic may have killed one-third of the European population. The Black Death is seared into the collective memory of European civilization, reflected in the art and culture of the Middle Ages.

Closer to our time, the 1918 Spanish flu pandemic was so deadly because of the virulence of the virus. “The Spanish flu distorted population in some places,” Hays said. “It was in India where it was most savage. The birthrate was suppressed for years. [The outbreak] was unusual in that it affected young adults. Usually epidemics affect the young and old. AIDS did something of the same thing in Africa where young adults fell victim to the disease.”

Vaccines have stanched the spread of afflictions such as polio. The first major polio epidemic in the United States was in 1916 and peaked in 1952. Three years later, Dr. Jonas Salk developed a vaccine to combat polio, which has been virtually eradicated in the United States.


To compile a list of the worst outbreaks of all time, 24/7 Wall St. reviewed reference materials, books on the subject of epidemics and pandemics, media sources, and health and medical organizations websites. We recognize that death totals for these epidemics are estimates.