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 afebrile infection to a rapidly progressive pneumonia. Cases of infection occurred mostly in children and young adults. The characteristics associated with adults were the presence of less comorbidity, more extensive respiratory compromise, and intensive care unit admission. These were key features of the clinical presentation of patients with novel H1N1-associated pneumonia, compared with other viruses, and in particular seasonal influenza pneumonias. Bacterial coinfections, particularly Streptococcus pneumoniae, increased the severity of illness and consumption of health resources. On December 5, 2009, 208 countries had reported cases and over 10,000 deaths had been registered. On August 10, 2010 the World Health Organization announced that the H1N1 pandemic had moved into the postpandemic period and reported a total of 18,500 confirmed deaths worldwide. Patients from the postpandemic period had an unexpectedly higher mortality rate and showed a trend toward affecting a more vulnerable population, in keeping with more typical seasonal viral infection. Early use of noninvasive ventilation in less severe cases of acute lung injury showed an increasing rate of success and required shorter ventilation time, shorter intensive care unit stay, and shorter hospital stay. Copyright © 2012 by Lippincott Williams & Wilkins.