How does the flu turn fatal?

You can certainly feel like death when you get the flu, which often lasts for a dreadfully snotty and cough-wracked week. But in some rare cases, the flu can actually be fatal. What happens in the body when the flu turns fatal? How does a relatively common sickness actually end your life?

When the flu virus enters your body, it triggers your immune response — and in some lethal cases, that response pummels not just the virus but the body too. The influenza virus hijacks human cells in the nose and throat to make copies of itself. This hoard of viral beasties triggers the immune system to send battalions of white blood cells, antibodies and inflammatory molecules to eliminate the threat, according to Scientific American. Usually, that process works to heal the body. But sometimes, the immune system’s reaction is so strong, destroying so much tissue in the lungs that they can no longer deliver enough oxygen to the blood, which in turn causes hypoxia and death.

In a particularly gruesome way to go, people with the flu can experience “multiple organ failure” throughout their bodies. Other complications can set in, such as inflammation of the heart, brain or muscle tissues, or sepsis, all of which can be life-threatening, according to the Centers for Disease Control and Prevention.

Perhaps the most terrifying part is that the flu likes to break its own rules. During the infamous 1918 epidemic, an estimated 500 million people became infected, according to the CDC. There were about 50 million deaths worldwide.