Hillary Clinton's health has suddenly erupted as a serious issue after she became ill and departed early from a 9/11 memorial ceremony in New York Sunday. Her doctor reported that the Democratic presidential nominee had pneumonia, according to CNN and Politico, after her spokesman said only that Clinton "felt overheated" and had to depart. Clinton spent some time at her daughter Chelsea's nearby apartment and quickly felt better, the spokesman said.

A video appeared to show Clinton's knees buckling as she tried to enter a vehicle as she left the ceremony. The video, taken from a distance, also seemed to show Clinton nearly collapsing and being helped into the vehicle by people around her, apparently members of her security detail. Reporters were left behind.

Even though Clinton, 68, was described as feeling overheated, the temperature at the ceremony was in the high 70s, not oppressively hot.

The timing of the announcement that she had pneumonia raised questions about how candid Clinton has been about her medical condition. Lisa Bardack, her doctor, announced the diagnosis on Sunday after Clinton left the public ceremony, but the doctor said she had made the diagnosis Friday and no announcement was immediately made at that time.

The pneumonia revives questions about the Democratic presidential nominee's condition at a crucial time, two months before Election Day. Republican candidate Donald Trump, 70. has raised doubts about whether she has the stamina and energy to be president. Former Mayor Rudy Giuliani of New York, a prominent Trump surrogate, has said Clinton appears to be very sick.

Central to the questions about Clinton's health is the fact that she fainted and suffered a concussion at the end of her term as secretary of state in 2012. It took her several weeks to recover, her aides said. She was found to have a blood clot during that period. Clinton critics argue that she may have suffered permanent harm, including a possible lessening of her cognitive abilities.

Two coughing fits over the Labor Day weekend, both recorded on video, added to the questions about her health.

Clinton earlier released a letter from her doctor insisting that her health was good. Trump also has released a letter from his doctor that Trump's health is good, but with even less detail than what was contained in Clinton's letter. Neither candidate has agreed to release their full medical records.

Presidents have suffered from many physical ailments while in office, and some of these ailments have affected their ability to carry out their duties. Sometimes, these ailments were kept secret.