-- A 26-year-old newlywed's death from a flu complication is a sobering reminder that the flu can kill even healthy people with no underlying medical conditions.
Katie McQuestion started feeling sick on Monday, and she died on Friday of sepsis, her mother told ABC News. She was just married in September.
"Someone can look good one day and not look good the next day," said Dr. Frank Esper an infectious diseases expert at UH Case Medical Center in Cleveland, Ohio. "Anyone can get really sick from the flu and anyone can die from the flu."
As the flu season goes into full swing, emergency departments across the country expect "a huge volume" of patients with the flu and other viruses, Esper said. Most of the time, they have nothing to worry about, but "a very small percentage" come down with severe or life-threatening illnesses and complications.
In Nevada, hospitals are so full of flu patients that public health officials are asking people without emergencies to seek care somewhere else, according to several news outlets in the state.
Most people who die from complications of the flu tend to die of pneumonia, an infection in the lungs, Esper noted.
"Influenza basically opens the back door for other germs to cause really bad diseases," he said. "Your immune system is all focused on fighting off influenza that a germ like staph or strep sneaks in through the back door and causes really, really bad pneumonia or can get into the blood and cause sepsis."
According to the U.S. Centers for Disease Control and Prevention, 53,826 people died from the flu and pneumonia in 2010.
If you have an underlying condition that makes you more likely to get severe influenza -- such being immuno-compromised, having asthma or being very young or very old -- you should call your doctor as soon as you start experiencing flu symptoms, Esper said. Your doctor can prescribe an antiviral medication that will keep your illness from worsening, he said.
If you don't have the above conditions, here's when you or your family should call your doctor or take you to the hospital: