Did Prince Die from the Flu? How the Virus Kills
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Exactly what caused the death of singer-songwriter Prince today (April 21) at age 57 is still unknown; however, some reports suggest that he may have died of the flu, which he was hospitalized for several days ago, his publicist told news sources.

People often underestimate how deadly the flu is, said Dr. Amesh Adalja, an infectious-disease specialist and a senior associate at the University of Pittsburgh Medical Center's Center for Health Security. Adalja has not treated Prince and was not involved with his case.

"Flu is a deadly infectious disease that kills thousands of people every year," Adalja told Live Science. But because these deaths occur mostly in the very young and the old, people don't really think about it in terms of a 57-year-old, he said. However, dying of the flu is something that can happen easily under certain circumstances, he said. [10 Celebrities with Chronic Illnesses]

There are several different ways that flu can kill someone.

One way is directly from the virus, Adalja said. The flu virus can cause such overwhelming inflammation in a person's lungs that they die due to respiratory failure, he said. The influenza virus can damage the lungs to the point that it becomes impossible for enough oxygen to pass through the lung tissue and into the person's blood, he said. When death occurs this way, it usually happens very quickly, he said.

People can also die of a secondary bacterial infection, Adalja said. When someone is sick with the flu, they may become more vulnerable to a bacterial infection that can cause pneumonia, he said. During the 1918 flu pandemic, the majority of deaths were caused by such secondary bacterial infections, he added. Death from secondary infections take a bit longer than death from the virus itself — usually about a week or so — because it takes a while for the secondary infection to set in, he said.

Flu can lead to deaths in other ways as well, Adalja said. For example, people who are sick with the flu can experience organ failure throughout their body. Doctors call this "multiple organ failure,” and it can lead to death, he said. 

The number of deaths from the flu in the U.S. varies widely from year to year, according to a study that looked at flu deaths between 1976 and 2007. Flu-associated deaths in a year ranged from a low of about 3,000 to a high of about 49,000, the study found.

Follow Sara G. Miller on Twitter @SaraGMiller. Follow Live Science @livescience, Facebook & Google+. Originally published on Live Science.