Newly elected Pope Francis is two years younger than Pope Benedict XVI was when he was elected in 2005, but the 76-year-old from Argentina still has at least one health issue: He has only one lung.

Pope Francis, formerly known as Cardinal Jorge Bergoglio, had one of his lungs removed when he was a teenager because of an infection, according to the Associated Press.

"Obviously, this was a success because here he is at age 76," said Dr. William Schaffner, chairman of preventive medicine at Vanderbilt University Medical Center in Nashville, Tenn. "So whatever they did got him over that precarious period."

Pope Francis is revealed at the Vatican on March 13, 2013. (Credit: ABC News)

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Schaffner, a former president of the National Foundation for Infectious Diseases, said there are several reasons the new pope could have had his lung removed about six decades ago:

1.) Pope Francis could have had tuberculosis.

"Back when he was a young person, there was not yet widespread antibiotic drug therapy, and it is possible that he had substantial involvement of a lung or part of a lung and had to have it removed," Schaffner said. "That was a pretty standard treatment in the pre-antibiotic drug era."

2.) He could have had a complication of whooping cough, or pertussis.

"Whooping cough can cause disease of the bronchial tube and can cause chronic infection," Schaffner said.

3.) He could have had pneumonia and developed complications.

"Once again, this was before conventional antibiotics were widely available, and so they may have had to treat this complication with surgery by taking out all or part of his lung," Schaffner said.

4.) Finally, he could have been born with a congenital lung defect that got infected.

"They could not eradicate the infection and, once again, did not have all the antibiotic supportive care that we have today," Schaffner said. "They just cut out infection."

Although these infections would have been more common in a middle-aged person than a teenager, Schaffner said they're the most likely possibilities given the limited information about the lung removal.

Asked whether a person can survive with one lung, Schaffner said, "Easily." - lungs are redundant, so people who have one removed are able to live normal lives.

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"Many people have gone on to live perfectly normal lives, even to engage in tennis, hiking and jogging with one lung," Schaffner said. "It's like being able to live with only one kidney."

But if Pope Francis gets a disease, he only has one lung to spare.

As people age, they become more vulnerable to lung infections, such as pneumonia and bronchitis, according to the National Institutes of Health. People older than 65 are especially likely to develop such infections. Their risk increases if they have preexisting conditions such as a weakened immune system or heart disease.

"Pneumonia is, in fact, one of the more common respiratory conditions as you age," said pulmonologist Dr. Greg Martin, who teaches at Emory University and specializes in critical care. "As a matter of fact, it's one of the more common causes of death."

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Martin said there's very little data about the prognosis of people with one lung and these infections because having one lung is so rare. But he suspects it would be a complicating factor. People with decreased lung capacity, such as those with emphysema, have a more difficult time recovering from lung infections because they don't have "reserve" healthy lung capacity to help them breathe while the infection heals.

"For someone like him, one lung is a potential complication," he said. "If someone has reduced lung capacity - one lung, underlying lung disease - they're more susceptible to more severe pneumonia and more serious complications from pneumonia."