<p>Surgical masks provide a measure of protection against a killer flu for American baseball players in 1918. That year—the final year of World War I—the Spanish flu took 50 million lives worldwide, at least three times as many as during the war.</p>

Influenza Fears, 1918

Surgical masks provide a measure of protection against a killer flu for American baseball players in 1918. That year—the final year of World War I—the Spanish flu took 50 million lives worldwide, at least three times as many as during the war.

Photograph by Underwood and Underwood/Time & Life Pictures/Getty Images

Influenza

Find out how the seasonal flu can become a killer cold.

For most people, flu is a seasonal nuisance, announced by a fever, a headache, and a sore throat. A few days to a week and you're over it. However, the virus can be a lot more serious. Influenza, as it's properly known, causes up to half a million deaths a year worldwide. And less than a century ago it became a global catastrophe, striking a fifth of the planet's population and claiming more than 50 million lives. Experts say we can expect a similar outbreak in the future.

Among the most common contagious human diseases, influenza usually strikes in winter, spreading through airborne droplets and particles emitted in coughs or sneezes. Between 5 and 15 percent of the population

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