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You can certainly feel like death warmed over when you get the flu, which often lasts for a dreadfully feverish, snotty and cough-wracked week. But in some cases, the flu can actually be fatal. Last year, influenza killed about 80,000 people, including 180 children, according to the Centers for Disease Control and Prevention (CDC).

Just what happens in the body when the flu turns fatal? How does a relatively common winter sickness actually end a person’s 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 horde 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 that it destroys so much tissue in the lungs that they can no longer deliver enough oxygen to the blood, which, in turn, causes hypoxia and death.

It’s also possible to die from a secondary bacterial infection, such as pneumonia. Death from such secondary infections usually occurs about a week or so after a person first gets sick, because it takes time for the secondary infection to set in, Live Science reports.

The flu can lead to death in other ways, as well. In a particularly gruesome way to go, people with the flu can experience “multiple organ failure.” Other serious 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 CDC.

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, one-third of the world’s population, became infected, notes the CDC. There were about 50 million deaths worldwide and about 675,000 in the United States. The deaths were even more shocking because they tended to occur not in children or the elderly, whom we usually think of as most vulnerable, but in healthy people from 20 to 40, people in the prime of their life.