Flu Is Sending More and More People to Hospital By LINDSEY TANNER    CHICAGO (AP) -- The number of people hospitalized in the United States because of the flu has climbed substantially over the past two decades to an average of more than 200,000 a year, in large part because of the aging of the population, a government study found.    Severe cases of the flu can result in pneumonia, dehydration or a worsening of chronic medical conditions, and can be life-threatening - especially for the very old or the very young...