 Unlike most strains of influenza , H1N1 does not disproportionately infect adults older than 60 years ; this was an unusual and characteristic feature of the H1N1 pandemic . Even in the case of previously very healthy people , a small percentage will develop pneumonia or acute respiratory distress syndrome ( ARDS ) . This manifests itself as increased breathing difficulty and typically occurs 3 – 6 days after initial onset of flu symptoms . The pneumonia caused by flu can be either direct viral pneumonia or a secondary bacterial pneumonia . In fact , a November 2009 New England Journal of Medicine article recommends that flu patients whose chest X @-@ ray indicates pneumonia receive both antivirals and antibiotics . In particular , it is a warning sign if a child ( and presumably an adult ) seems to be getting better and then relapses with high fever , as this relapse may be bacterial pneumonia . 
