Abstract
During the spring of 2009, a pandemic influenza A (H1N1) virus emerged and spread globally. Preliminary comparisons with seasonal influenza suggest that pandemic 2009 influenza A (H1N1) disproportionately affects individuals of younger ages and causes a generally mild disease. However, based on clinical studies in Mexico, North America, and the southern hemisphere, the virus has caused an epidemic of critical illness, with some patients developing severe acute respiratory distress syndrome (ARDS). A proportion of these patients with ARDS have even been treated using extracorporeal membrane oxygenation (ECMO). Since the fall of 2009, the number of critically ill patients has been increasing in Japan, although the number is much lower than in foreign countries. In this article, we focus on the characteristics of the novel 2009 influenza A (H1N1) virus and the clinical features of patients infected with this virus. Also, we refer to the recent knowledge describing the molecular mechanisms by which the influenza virus can cause critical illnesses, such as ARDS, in humans.
