U.S. Air Force Staff Sgts. Natasha Swartzendruber (left center) and Cherie Gregory, 59th Medical Operations Squadron medical technicians, prepare patients for their annual flu vaccines inside the Atrium at Wilford Hall Ambulatory Surgical Center on Joint Base San Antonio-Lackland, Sept. 4, 2012. An estimated 5-20 percent of Americans come down with the flu every year leading to more than 200,000 hospital visits.