By Elizabeth Cohen
CNN Medical Correspondent
(CNN) -- One look at her photo, and you can't help but ask: How could someone so young and vibrant die so quickly from an infection?
Brazilian model Mariana Bridi da Costa died from a sepsis infection.
Brazilian model Mariana Bridi da Costa was a healthy 20-year-old when doctors told her she had a urinary tract infection, her family says. The infection spread, and after amputating her feet, doctors thought they had the situation under control, according to a blog run by a family friend.
"She's alive, [she] will survive," Renato Lindgren wrote on the blog on January 20. "She can eat well, visit the sea, swim, travel, talk with her friends and family, marry and have a baby. She has a full and beautiful life ahead."
Four days later, da Costa was dead.
Sepsis -- the body's inflammatory response to an infection -- really can kill that quickly, according to Dr. Kevin Tracey, author of a book about sepsis called "Fatal Sequence: The Killer Within."
"This isn't a one in a million case," says Tracey, chief executive officer of the Feinstein Institute for Medical Research in Manhasset, New York. "When an infection reaches a certain point, this can happen in a matter of hours."
Sepsis usually starts out as an infection in just one part of the body, such as a skin wound or a urinary tract infection, Tracey says. For example, Muppets creator Jim Henson died in 1990 from a case of sepsis that started out as pneumonia, an inflammation of the lungs. He was 53.
Most of the time, simple, localized infections remain just that: easy to treat and in one part of the body. Why some infections rage out of control and shut down vital organs is a mystery, but experts say it rarely happens in young, healthy people, like da Costa.
"You can ask, 'Why her?' but really no one knows why her," Tracey says. "It might have something to do with her immune system. It might be about her genetics."
The Mayo Clinic sees about 100 cases a year of young, healthy people who develop sepsis, says Dr. Priya Sampathkumar, an infectious disease specialist at Mayo. With treatment, which usually involves antibiotics and sometimes draining of the wound, about 75 percent survive.
Sampathkumar says the key is to keep an eye on even such simple infections as a small skin wound. A fever, a dramatic shift in blood pressure, rapid breathing and extreme confusion are all signs that someone needs quick medical help, she says.
"You need to watch it," she says.
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Dr. Carl Flatley said he had no idea what to watch for when his 23-year-old daughter, Erin, developed sepsis in 2002. He says she went into the hospital for a minor hemorrhoid procedure and five days later was dead from sepsis. "It's a horrible death," he says.
Two years later, knowing the signs of sepsis saved his own life, says Flatley, a dentist in Dunedin, Florida. He fell ill, and an emergency room doctor said he had a urinary tract infection and sent him home.
But Flatley said he suspected it was much more than a UTI. "I felt very sick. My testicle was sore. I told him I was concerned I had sepsis and I refused to go home. He got very irritated with me," Flatley says. But in the end, Flatley was admitted to the hospital, where the infection had become so severe that doctors were forced to remove his right testicle.
Flatley started The Sepsis Alliance to educate others about sepsis, where he tries to walk a fine line. "You don't want to panic people. We all get infections and, thank God, most of them heal," he says. "So this is what I tell people: If you're feeling bad all over and have a high temperature and either high or low blood pressure, those are all indications that your whole system has been infected. It could be sepsis."
He advises getting medical help immediately, and to specifically mention that you're concerned you might have sepsis. "You don't want to take any chances," he says.