All patients were young male soldiers with a mean age of 20 years (age range: 19 to 22 years). Fifteen of the 18 patients confirmed with the pandemic H1N1 2009 influenza virus infection were from the same barrack, and they visited the emergency department due to acute febrile respiratory illness from July 18 to July 20, 2009. The other three patients were from different barracks and were admitted with suspected cases of rhabdomyolysis after a long distance march (n = 1), acute gastroenterocolitis (n = 1), and acute febrile respiratory illness with leukocytopenia and thrombocytopenia (n = 1). All three patients came into contact with one or more of the previously-mentioned patients lying in nearby beds in our hospital.