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A 10-year-old Connecticut boy died of flu. So did a 21-year-old bodybuilder, and 4-year-old Jonah Reiben of Dayton, Ohio.

These are not the usual sick and elderly people who die from influenza. But every year, flu carries away perfectly healthy young adults and children, and tens of thousands of people over 65.

Influenza virus CDC

How does flu kill, and why does it sometimes kill so quickly?

Doctors who study the body’s immune response say there are three main reasons: co-infection with another germ, usually bacteria such as strep; aggravation of existing conditions such as heart disease and asthma; and a so-called cytokine storm, marked by an overwhelming immune system response to infection.

Related: Flu doesn't spare vulnerable kids

Sometimes this can happen very fast. During the 1918 “Spanish flu” pandemic that killed up to 50 million people a century ago, many people were reported to have died within hours of showing their first symptoms.

Researchers who have gone back and re-examined tissue samples, and read reports from the time, believe most deaths were caused by co-infection with another germ. But many of the healthy young men and women who died quickly of flu that year more likely succumbed to cytokine storms.

The human immune system has a load of weapons to throw out against infections, including cytokines produced by a variety of immune system cells.

“Those substances work to stop the virus from spreading,” said Dr. Amesh Adalja, an infectious disease specialist and senior scholar at the Johns Hopkins University Center for Health Security.

Related: Tsunami of flu activity covers entire continental U.S.

They cause the typical “flu-like symptoms” that bring misery from flu and other infections.

“The muscle aches, the fever — all of that is the result of the immune system responding to the virus,” Adalja said. That’s why so many diseases cause similar symptoms: it’s the body’s response, not the particular invader, that’s to blame.

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But different people have differently composed immune systems.

“In certain individuals there can be a very pronounced immune response that can result in a lot of damage to the cells in your body including the cells in the respiratory tract," Adalja said.

When a virus is new, like the 1918 strain of H1N1 and the 2009 H1N1 “swine flu”, it usually kills far more people. One theory is that the immune system can become overwhelmed by the never-before-seen invader and sends so many troops to fight it that perfectly healthy tissue in the lungs and other organs gets killed, too.

People who die from “bird flu” viruses, such as H5N1 or H7N9, also seem to die via an over-the-top immune response.

Related: Here's why you need a flu shot every year

And these newer viruses also tend to kill younger people, perhaps because the older population may have been exposed to a distant relative of the virus in the past. The H1N1 flu virus killed 282 U.S. children in 2009-2010, the Centers for Disease Control and Prevention says. It may have infected 61 million people.

Now it’s just part of the annual flu mix and while it is circulating and killing some people this year, it’s the H3N2 strain that is suspected of causing most problems this time around.

One experiment showed that certain “new” genes in these never-before-seen viruses help them thrive deep in the lungs, which can cause pneumonia and might provoke an overwhelming immune response.

While a few people seem to die within hours or days, flu can cause lingering sickness in others. Then they become susceptible to other infections, such as streptococcal or staphylococcal bacterial infections.

These secondary infections can damage organs, cause pneumonia or get into the bloodstream, causing another kind of immune system overreaction called sepsis.

So far this season, flu has killed 30 children, according to the latest CDC data. Last season, 110 children died from influenza in the U.S.

The CDC doesn’t precisely count adult flu deaths, in no small part because it just kills so many. Every year, flu kills 12,000 to 56,000 people and sends as many as 700,000 to the hospital.

Related: H1N1 swine flu hit Americas especially hard in 2009

CDC estimates flu deaths by looking at how many more people than usual died of flu and pneumonia, but even those calculations miss people who may have died from flu complications, such as a heart attack set off by a bout of flu.

For patients with asthma or other lung conditions, flu is just one more problem for the lungs to cope with.

“They are already having breathing difficulties. It can put them into a spiral very quickly where their breathing gets compromised,” Adalja said.

Patients with diabetes already have a damaged immune response, so they also are more susceptible to flu.

Related: Death of teen athlete is a reminder that flu can kill anyone

And pregnant women have a double risk. “Pregnant women are in a state of immunosuppression because the immune system is trying not to reject the fetus,” said Adalja. So the virus can get further, faster in their bodies.

Plus their lungs are compressed by the fetus, so they have less breathing capacity. Humans need a certain level of oxygen and if blood oxygen levels fall too far, they enter a state called hypoxia. Hypoxia can cause organ damage within minutes.

That’s why bluish skin or difficulty breathing is an emergency that requires immediate medical care.

The best defense against flu, the CDC, FDA, pediatricians and other health experts agree, is a flu vaccine. Just about everyone over the age of 6 months should get one and it's still not to late to do it.

And flu is spread by droplets that can linger on surfaces such as countertops, which is why hand-washing is so important. It also spreads via sneezing and coughing and, perhaps, may float in the air on tiny droplets emitted by simple breathing.