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Skip Navigation LinksPennsylvania Department of Health > My Health > Emergency Preparedness > Pandemic > About Flu Pandemics


Each year, we experience seasonal flu during the cooler time of the year. Seasonal flu strains are similar, but not identical to, strains from previous years. But sometimes a new, unrelated flu strains appear. When this happens, a flu pandemic can occur.

A flu pandemic is a widespread occurrence of a new type of the flu. It happens with little warning and can appear at any time of the year. Since it is a new virus, people have little or no defense against it. As a result, the virus spreads easily and quickly from person to person around the world, causing serious illness and death1. There have been four flu pandemics in the last 100 years, ranging from very severe (approximately 50 million deaths in 1918) to less severe (an estimated 10,000 deaths in 2009).

A flu pandemic remains a constant threat. Health organizations around the world routinely check for flu strains that may lead to pandemics.

Due to modern travel, a flu pandemic could spread more quickly than ever and affect more people.