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NEW YORK -- Hillary Clinton's diagnosis of pneumonia is a serious concern, but something from which she soon should recover, several doctors and medical experts said Sunday.

Clinton, 68, unexpectedly left a 9/11 anniversary ceremony in New York after she became "overheated and dehydrated," her doctor said. Clinton went to her daughter's nearby apartment for a short stay, and emerged before noon to tell reporters, "I'm feeling great."

Several hours later, Clinton's physician said the Democratic presidential nominee was diagnosed on Friday with pneumonia. "She was put on antibiotics, and advised to rest and modify her schedule," said Dr. Lisa R. Bardack, an internist who practices near Clinton's suburban New York home.

Bardack added in a statement that Clinton, after an exam Sunday afternoon at her home, "is now rehydrated and recovering nicely."

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WHAT IS PNEUMONIA?

Pneumonia is an infection of the lungs. Often it's a bacterial infection that sweeps in after a cold or flu virus. Each year, about 1 million people in the United States seek hospital care because of pneumonia, and it causes tens of thousands of deaths, according to the Centers for Disease Control and Prevention.

A presidential candidate is at high risk for such an infection, said Dr. William Schaffner, an infectious disease expert at Vanderbilt University.

"Candidates are constantly out in enclosed spaces, face to face with myriads of people," he said. "It's an ideal opportunity for the transmission of a respiratory virus."

Added Dr. Stephen Hargarten, head of emergency medicine at the Medical College of Wisconsin: "I'm not surprised to hear this happen to someone engaged in the kind of frenetic schedule she has."

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HOW IS IT TREATED?

Pneumonia is commonly treated quickly and effectively with antibiotics. Speaking generally and not about Clinton's case, Schaffner said patients with a mild pneumonia can recover with antibiotics, a few days of rest and good hydration. That's especially true of someone who does strenuous work, such as a rigorous campaign schedule.

Based on the available information, he said, "this should not in any way impede her function going forward."

But Clinton does need to take it seriously, some experts said.

People over age 65 have a harder time returning to normal than do younger patients. Many people her age need a week or more to recover from even a mild case of pneumonia, said Dr. Sharon Bergquist, an Emory University assistant professor of medicine who specializes in internal medicine.

"The body needs rest. The more she pushes, the harder it is for her to recover," she said.

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CLINTON'S RECENT HEALTH ISSUES

Clinton had a coughing fit while campaigning in Cleveland early last week. It was dismissed by her aides as allergies and by Clinton herself at that moment as stemming from "talking so much."

It's possible Clinton may have assumed that symptoms from an earlier viral infection were due to allergies, Schaffner said.

Clinton takes antihistamines, which can "dry you out," and dehydration leads to a person being susceptible to heat exhaustion, Bergquist said.

Add in the possibility of fever, shortness of breath or other possible symptoms from pneumonia, and you have a constellation of factors that could have explained her feeling weak on Sunday, she said.

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CLINTON'S MEDICAL HISTORY

Last year, Clinton's campaign released a letter from Bardack attesting to the former secretary of state's good health. The most notable events in Clinton's medical record included deep vein thrombosis - or a blood clot, usually in the leg - in 1998 and 2009, a broken elbow in 2009 and a concussion in 2012.

Clinton got a stomach virus while traveling in 2012 that left her so dehyrdated that she fainted. She had a concussion that fall, and doctors treating her found a blood clot in a vein in the space between the brain and the skull behind her right ear. Clinton spent a few days in New York-Presbyterian Hospital for treatment and took a monthlong absence from her role as secretary of state.

Bardack said testing the following year showed "complete resolution" of the concussion's effects, including double vision, for which Clinton wore glasses with specialized lenses to address.

Other details from Bardack's letter included:

— Clinton's blood pressure was 100 over 65. Her total cholesterol was 195; her LDL or "bad" cholesterol was 118, and her HDL or "good" cholesterol was 64 - all within healthy levels and not signaling the need for any medications.

— She had full cardiac testing, including an ultrasound exam of arteries in her neck, and all was well.

— Clinton has been diagnosed with hypothyroidism, a common condition in women older than 60, in which the thyroid gland doesn't produce enough of certain important hormones.

— Clinton's current medications include Armour Thyroid, a thyroid hormone replacement, and antihistamines, vitamin B12 and a blood thinner named Coumadin.