Since 1900, health authorities have identified three influenza pandemics, which are widespread global outbreaks. Two other ominous outbreaks did not reach that level.
1918-20 Spanish flu: The pandemic against which all others are measured. It infected up to 40 percent of the world's population and killed more than 50 million. About one victim in 15 died, some within 24 hours of their first symptoms. The illness originated in China or the United States, but became known as Spanish flu because Spain had less censorship during World War I, so cases were heavily publicized.
1957-58 Asian flu: This was the first flu strain to be identified early in the pandemic, which allowed the creation of a vaccine to soften its blow.
Its first toll affected schoolchildren, but a second wave in early 1958 hit the elderly hard. About 2 million people died worldwide, including 69,000 in the United States.
1968-69 Hong Kong flu: Pandemic first identified in Hong Kong, where 500,000 people were infected. An estimated 1 million people died worldwide, including about 33,000 in the United States. This virus shared genetic characteristics with the 1957 Asian flu, which lessened its impact because people had built immunities. It peaked in the United States during Christmas vacation, dampening its spread among schoolchildren.
1976 Swine flu scare: Identified when about 200 soldiers at Fort Dix, N.J., got sick and one died. Genetic strands showed that it originated in pigs and carried several similarities to the 1918 pandemic virus. Mass inoculations started, then halted after several people apparently died from the vaccine. The flu never spread beyond the Fort Dix cases, leading to criticism that health authorities had overreacted.
2003 to present, Bird flu: The lethal virus spread mainly among birds, but with limited transmission to humans. Mortality rate has been 60 percent. Officials worry a mutation will allow it to spread through human-to-human contact, setting off a pandemic.
Sources: U. S. Centers for Disease Control and Prevention, Centers for Infectious Disease Research & Policy, University of Minnesota