This retrospective study will analyse all PCR-proven H1N1 cases from the influenza season 2009/2010 treated within the styrian "LKH hospital network". For this purpose all PCR-positive case files are reviewed in Medocs (electronic patients database) and data regarding clinical presentation, laboratory and radiological findings, treatment, outcome and preexisting underlying chronic illnesses will be systematically collected. In a second step the same data collection will be performed in a group of PCR-negative patients, which were tested during the influenza season 2009/2010 for influenza-like illness. The data from the PCR-positive group will be compared to the data from the PCR-negative group. We expect significant differences between the proven and unproven group regarding the primary presentation at hospital. Based on these results a clinical score will be developed. This score should improve H1N1 case identification in emergency departments, even if specific diagnostic test are negative (rapid antigen testing) or still pending (PCR), reduce the number of missed hospitalized H1N1 infection and optimize the decision making process in emergency departments regarding which patient has to be admitted with infection control measures and which not, and as infection control measures are expensive (face masks, gloves) and limited (isolation room), such a score should also reduce unnecessary expense. To evaluate the sensitivity and specificity of the score a prospective study will follow in the influenza season 2010/2011.