Pandemic influenza A (H1N1) virus emerged in Mexico during the spring of 2009 and spread rapidly and caused significant strain on health systems worldwide. The clinical picture of the pandemic influenza A (H1N1) virus ranges from a self-limiting a-febrile infection to a rapidly progressive pneumonia. The presence of less co-morbidity, more extensive respiratory compromise, and ICU admission are key features of the clinical presentation of patients with novel H1N1-associated pneumonia compared with other viruses and in particular seasonal influenza pneumonia. Bacterial co-infections, particularly Streptococcus pneumoniae, increased the severity of illness and consumption of health resources. Patients from the post-pandemic period had an unexpectedly high mortality rate and showed a trend towards affecting a more vulnerable population, much like more typical seasonal viral infection. Early use of non-invasive ventilation in severe cases of acute respiratory failure required shorter ventilation time as well as shorter ICU stay and hospital stay.