Most flu deaths are caused by the body’s own immune system reacting either to the influenza virus or to a secondary infection, usually from bacteria. The immune system operates in a delicate balance: the immune response must be aggressive enough to eliminate pathogens, while limiting collateral damage to the body’s own tissues. In most healthy adults, this balance is correct and they recover within a few weeks at most. In others, inflammation may damage the lungs, cause them to fill with fluid, and impair oxygen exchange. A wide range of other complications are possible, such as septic shock (a widespread inflammatory response which causes a life-threatening drop in blood pressure).