How the flu spreads

LOS ANGELES, CA - OCTOBER 26: Emily Moore administers an inoculation as free flu shots are given to people over 50 by the Los Angeles County Department of Health Services on October 26, 2005 in Los Angeles, California. An estimated 60 million flu shots will be given in the United States this season. (Photo by David McNew/Getty Images)

How the flu spreads

Declassified

National security expert Mike Rogers discusses the likelihood of a major flu outbreak.

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CNN's Original Series 'Declassified' 54 videos

LOS ANGELES, CA - OCTOBER 26: Emily Moore administers an inoculation as free flu shots are given to people over 50 by the Los Angeles County Department of Health Services on October 26, 2005 in Los Angeles, California. An estimated 60 million flu shots will be given in the United States this season. (Photo by David McNew/Getty Images)

How the flu spreads

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