This weekend, Rick Santorum left the campaign trail to be with his 3-year-old daughter, Isabella, who was hospitalized with pneumonia. Fortunately, Bella made what Santorum called "a miraculous turnaround," and the former Pennsylvania senator resumed his campaign for the 2012 Republican presidential nomination on Monday. However, Bella's condition remains a constant point of worry for the Santorum family.

Bella has Trisomy 18, a genetic disorder that gives her an extra copy of the 18th chromosome. According to the Trisomy 18 Foundation, the extra chromosome can cause life-threatening abnormalities in the heart, brain, stomach and other internal organs.

There is a very high mortality rate for infants born with the disorder -- about 50 percent of babies carried to term are stillborn, and less than 10 percent will reach their first birthday. Females are about five times more likely than males to be live-born. A small number of individuals (mostly women) with Trisomy 18 have reached their 20s and 30s.

Robert Marion, chief of genetics and developmental medicine at the Montefiore Medical Center's Children's Hospital in New York told the Washington Post that Trisomy 18 is "almost always diagnosed prenatally, and those pregnancies are almost always terminated."

Many children like Bella have congenital heart defects as a result of the disorder, meaning it is much more difficult for her heart to pump blood to the rest of her body. This can cause fluid to back up, and leave her more susceptible to infections like pneumonia.

ABC News reported Sunday evening that Santorum addressed Bella's hospitalization in a tele-townhall with Minnesota voters. "I know how she got through it," he said. "It was with the hands of these doctors and the prayers that guided those hands."

Santorum, who has said his daughter's condition has led to her life being "measured in days and weeks," has addressed the conflict of having a seriously ill daughter at home while he campaigns across the country.

In December, ABC's Christiane Amanpour asked the former senator how he can justify leaving Bella while he campaigns, given his low poll numbers. "Well, I don't worry about the polls," he said.

"I worry about what I'm trying to do to be the best father and the best husband I can be. And obviously a big part of that is making sure that we have a country that respects her life, and a country that is free and safe and prosperous for all of my children."

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