How does the flu actually kill people?

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ONE Sunday in November 20-year-old Alani Murrieta of Phoenix began to feel sick and left work early. She had no preexisting medical conditions but her health declined at a frighteningly rapid pace, as detailed by her family and friends in local media and on BuzzFeed News. The next day she went to an urgent care clinic, where she was diagnosed with the flu and prescribed the antiviral medication Tamiflu. But by Tuesday morning she was having trouble breathing and was spitting up blood. Her family took her to the hospital, where x-rays revealed pneumonia: inflammation in the lungs that can be caused by a viral or bacterial infection, or both. Doctors gave Murrieta intravenous antibiotics and were transferring her to the intensive care unit when her heart stopped; they resuscitated her but her heart stopped again. At 3:25 P.M. on Tuesday, November 28—one day after being diagnosed with the flu—Murrieta was declared dead.
Worldwide, the flu results in three million to five million cases of severe illness and 291,000 to 646,000 deaths annually, according to the World Health Organization and the U.S. Centers for Disease Control and Prevention; the totals vary greatly from one year to the next. The CDC estimates that between 1976 and 2005 the annual number of flu-related deaths in the U.S. ranged from a low of 3,000 to a high of 49,000. Between 2010 and 2016 yearly flu-related deaths in the U.S. ranged from 12,000 to 56,000.
But what exactly is a “flu-related death?” How does the flu kill? The short and morbid answer is that in most cases the body kills itself by trying to heal itself. “Dying from the flu is not like dying from a bullet or a black widow spider bite,” says Amesh Adalja, an infectious disease physician at the Johns Hopkins University Center for Health Security. “The presence of the virus itself isn’t going to be what kills you. An infectious disease always has a complex interaction with its host.”
After entering someone’s body—usually via the eyes, nose or mouth—the influenza virus begins hijacking human cells in the nose and throat to make copies of itself. The overwhelming viral hoard triggers a strong response from the immune system, which sends battalions of white blood cells, antibodies and inflammatory molecules to eliminate the threat. T cells attack and destroy tissue harboring the virus, particularly in the respiratory tract and lungs where the virus tends to take hold. In most healthy adults this process works, and they recover within days or weeks. But sometimes the immune system’s reaction is too strong, destroying so much tissue in the lungs that they can no longer deliver enough oxygen to the blood, resulting in hypoxia and death.
In other cases it is not the flu virus itself that triggers an overwhelming and potentially fatal immune response but rather a secondary infection that takes advantage of a taxed immune system.

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