Stories of flu deaths may have you especially on edge this season.The flu causes 290,000 to 650,000 deaths globally every year, according to the World Health Organization. This year’s flu season has been the worst since the 2009 swine flu pandemic, and hospital admission rates continue to rise. How does it become so deadly? The flu can be fatal as the body attempts to heal itself. The immune system may overreact and destroy tissue in the lungs, creating a shortage of oxygen to the blood.This can lead to hypoxia, a deficiency of oxygen reaching the tissues, and death. A second infection, often a strain of strep throat or a staph infection, can also cause damage as the immune system works overtime. A bacterial infection in the respiratory tract can spread to other parts of the body and blood. This could cause sepsis, which damages organs, leading to organ failure and death.The worst flu season took place a century ago, killing up to 50 million people.