Pandemic influenza is a global outbreak. An influenza pandemic occurs when a new flu virus emerges for which humans have little or no immunity, which allows the virus to spread easily from person to person worldwide.
Three influenza pandemics occurred during the 20th century: the 1918-'19 “Spanish flu,” the 1957-'58 “Asian flu,” and the 1968-'69 pandemic or “Hong Kong flu.” In 2009, the World Health Organization declared a global pandemic of H1N1 flu. This action was a reflection of the spread of the new H1N1 virus, not the severity of illness caused by the virus.
If a pandemic similar in virulence to the 1918 influenza strain were to occur, 2 million Americans could die and roughly 90 million would likely become ill. Worldwide, between 51 million and 81 million people could die, according to a study in Lancet.