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 rare cases, the flu can actually be fatal. Last year, influenza killed about 80,000 people, including 180 children, according to the US Centers for Disease Control and Prevention (CDC).

Just what happens in the body when the flu turns fatal? How does a relatively common illness actually end your life?

The sad truth is that 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.

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.

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.

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” throughout their bodies.

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.

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 ages 20-40, people in the prime of their life. – The Mercury News/Tribune News Service