(CNN) - This flu season is fierce and has already claimed the lives of at least 37 children in the United States, according to the Centers for Disease Control and Prevention.
There were 11,965 laboratory-confirmed flu-related hospitalizations reported from October 1 to January 20. The number of people infected with influenza is believed to be much higher because not everyone goes to their doctor when they are sick, nor do doctors test every patient.
Added to those scary stats, the World Health Organization estimates that annual flu epidemics result in about 3 to 5 million cases of severe illness globally and 290,000 to 650,000 deaths.
Although the fever and aches may feel terrible, most of us don't die from the flu. So how exactly does this common illness lead to so many dying?
"Influenza and its complications disproportionately affect people who are 65 and older. They account for 80% of the deaths," said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University.
But young children and people who have an underlying illness, such as heart disease, lung disease or diabetes, are susceptible to dying from the flu as well, he said. There are three ways adults can succumb:
"The usual flu death is a person who gets influenza, gets all that inflammation in their chest, and then has the complication of pneumonia," explained Schaffner, who added that this is a "long, drawn-out process."
Pneumonia is an infection that causes the small air sacs of the lungs to fill with fluid or pus. Though this is the most common route to death, flu can be fatal for more unusual reasons.
"Much of the systemic symptoms that any of us have with influenza -- the fever, the aches and pains, the sense of exhaustion -- all of those are part of (our body's) response to the virus," said Schaffner. The symptoms we experience are an inflammatory response to the immune system "soldiers" that our body sends to fight any pathogen, he said.
"Pushing the war analogy, we all know there is incidental damage that occurs during the course of a war," said Schaffner, and so the flu can also take a perfectly healthy person "and put them in the ER in 24 to 48 hours."
Flu stimulates an immune response in everyone's body, but for some people, this natural response can be "overwhelming," noted Schaffner. "Young robust people can have such an overwhelming response that it's called a cytokine storm." Cytokines -- proteins that are created as part of the inflammatory response -- create a "storm" in the body, explained Schaffner: "And this cytokine storm can actually lead to sepsis in the person."
Kyler Baughman, 21, is one example of that happening. He died unexpectedly in December at UPMC Presbyterian Hospital in Pittsburgh after a bout with the flu. Baughman, a college student, worked two jobs and often posted pictures of himself at the gym on social media. The cause of his death, as reported by the Allegheny County Medical Examiner, was influenza, septic shock and multiple organ failure.
Chances of a heart attack are increased sixfold during the first seven days after a flu infection, a new study published Wednesday in the New England Journal of Medicine found. The study looked at nearly 20,000 cases of flu in Ontario adults age 35 or older.
The risk may be higher for older adults, said Dr. Jeff Kwong, lead author of the study and a scientist at the Institute for Clinical Evaluative Sciences and Public Health Ontario. Heart attack occurs when blood flow to the heart is abruptly cut off; this is also called acute myocardial infarction.
Since a few days usually elapse between getting sick and getting a lab test, Kwong said "the increased risk is probably within the first 10 days or so after exposure to the virus."
The research, which identified 364 hospitalizations for acute myocardial infarction among the flu cases studied, also showed a stronger association for influenza B than influenza A. "We would have needed more cases to determine if the difference was real or just a chance finding," said Kwong.
Though the new study did not identify the reasons why flu might lead to heart attack, Kwong and his co-authors theorize that infectious illness may cause inflammation, stress and constriction of blood vessels, which increases blood pressure.
Threats to children
The overwhelming majority -- 99% -- of children under age 5 who die from flu-related illness are in developing countries.
Children in the developed world may not face such high risks, but they are still vulnerable if they develop flu. Sepsis resulting from flu can cause the death of very young children, said Dr. Flor M. Munoz, an associate professor of pediatrics and infectious diseases at Baylor College of Medicine.
"Children have different risks depending on their age," said Munoz, and the most worrisome ages are "infants in the first year of life and those under 5 years of age."
"What's different from adults is children have a lot of opportunities to not only be exposed to flu but also to spread the flu," said Munoz. In general, children are the first to get sick when flu season begins, mainly because they are in school and playing with others -- and spreading germs.
"They can be completely healthy and still have problems with the flu," said Munoz. "The flu shot doesn't offer the same protection as it does for adults."
This is due to the lack of "immunologic experience" that children have. The immune system in infants is "still developing and it has different responses, let's say, to new things," said Munoz. "Young children will not necessarily have the same response that older children, adolescents or adults have." The same is true for very old people, said Munoz: "That's just a normal way the immune system works."
However, the worry whenever a young child or infant gets flu symptoms, including fever, is that they might have a more serious infection occurring at the same time. "Young children at that age can have meningitis, pneumonia, bacterial infections, not necessarily flu-related," said Munoz. "One needs to be more cautious."
"Certainly, we do tend to see secondary infections," said Munoz. So a child will start with the flu and the irritation in their noses and throats leave them exposed to more germs and so they develop another bacterial infection --- ear infections, say, or sinusitis or pneumonia.
With the child's immune system already fighting the flu and then another bacteria on top of that, sepsis may be the result. These are the cases we hear on the news, said Munoz, "previously healthy children that don't feel well and in a day or two they die of some complication."
Another threat? Though children and adults experience the same symptoms when sick with the flu, children are more likely to get diarrhea and to vomit. This can lead to dehydration in infants and small children, Munoz said, and it can be life-threatening at such a young age.
What do parents need to know?
"Every year we're going to have the flu. Every year we have anywhere between 50 and 100 deaths of children from the flu," said Munoz, who is also a member of the Committee on Infectious Diseases for the American Academy of Pediatrics. "This is something to be taken seriously."
Parents can make sure their children are vaccinated, she said. "As a mother, if you have something at hand that can protect your child, why not?"
"It's a very safe vaccine -- it is not true that you can get the flu with the vaccine," she added.
Lynnette Brammer, head of the CDC's Domestic Flu Surveillance team, supported Munoz' view. "We want to continue to emphasize that there's still a lot of flu activity to come, people that haven't been vaccinated should still get vaccine," said Brammer. "We may be getting close to the peak of this wave, it's not unusual to have a second wave of influenza B come through."
The flu shot is admittedly imperfect, Schaffner said, but there are still benefits. "If you get the vaccine and you have a flu-like illness, it's likely the illness is less severe," he said. "Data show you're less likely to get pneumonia and less likely to die."
If a child, especially a small one, becomes sick, parents should visit a doctor or health care provider who may prescribe medication, said Munoz.
By treating illness, antiviral drugs become a second line of defense against serious consequences. While most otherwise healthy people will not need to be prescribed antiviral drugs, those who may benefit from these medications are "people who are high risk, the elderly, children under 2, pregnant women and people with chronic health problems," said Brammer.
Antiviral drugs are known to work best when started within two days of getting sick. Studies show these drugs, which rarely produce side effects, can lessen symptoms and shorten the time a person is sick by one or two days.
"It's a brisk influenza season and I think it will end up being a moderately severe one," said Schaffner. "We'll take any bit of protection and prevention we can get."
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