Epidemiologistsarebracedforthebigone:thestrainoffluthatrivalsthepandemicof1918-1919,whichkilledatleast20millionpeopleworldwide.Inrecentyears,wehaveexperiencedscareswithahostofnewinfluenzaviruses:birdflu,swineflu,Spanishflu,HongKongflu,H5N1,andmostre