This study has a number of limitations. First, our patient series was small, but it should be borne in mind that since these H1N1 positive patients represent a population of 350.000 residents. Furthermore, diagnostic tests with procalcitonine, urine antigen and blood cultures were given only upon suspicion of an infection. The respiratory capacity tests represented a small number of patients in each group, and so no correlations could be preformed between the four subgroups. In addition, the baseline condition of each patient was not evaluated; this was either because they did not have a respiratory underlying condition or because they were patients that admitted in our hospital for the first time.