Severe influenza is characterized by cytokine storm and multi-organ failure with edema. We found that the "influenza virus-cytokine-trypsin/MMP-9 cycle" in the endothelial cells is one of the key mechanisms of vascular hyperpermeability, the major pathogen of multi-organ failure. Upregulated TNF-alpha, IL-6 and IL-beta induce ectopic pancreatic trypsin and pro-MMP-9 in the endothelial cells and in various organs. Trypsin mediates the viral hemagglutinin processing, which is crucial for viral entry and multicycles of replication. In addition, trypsin is the most potent pro-MMP-9 convertase to form active MMP-9 and both proteases synergistically destruct matrix around blood vessels. In addition upregulated trypsin triggers through its receptor, PAR-2, the modification of cellular functions, such as increase in calcium mobilization and mitochondrial membrane permeability, suppression of ATP generation and loss of tight junction constituent, zonula occludens-1. High risk patients who have impaired mitochondrial fuel utilization will easy get into energy crisis, resulting in vascular hyperpermeability in severe influenza.