Once a disease becomes highly transmissible and spreads efficiently and rapidly to humans around the world, it is considered a pandemic. There have been 10 recorded pandemics over the past 300 years, three of which occurred in the last century: (http://www.cdc.gov/flu)
- In 1918-1919, the “Spanish Flu” or H1N1 sickened an estimated 20-40% of the world’s population. Millions of people died; 500,000 in the U.S. It spread rapidly; many died within days of infection. The attack rate and mortality was highest among adults 20-50 years old, although the reasons for this are uncertain.
- In 1957-58, the “Asian Flu” or H2N2 caused an estimated 2 million deaths worldwide, including about 70,000 in the U.S. Infection rates where highest among school children, young adults and pregnant women; the death rate was highest among the elderly. A second wave developed in 1958.
- In 1968-69, the “Hong Kong Flu” or H3N2 caused nearly a million deaths around the world and 34,000 in the U.S. Victims over age 65 were most likely to die. This virus returned in 1970 and 1972 and still circulates today.
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