Hillary Clinton appeared unsteady and and had to be helped to a waiting van during a memorial ceremony for victims of 9/11 in New York City on Sunday, giving her Republican opponent the ammunition he needed to continue his recent line of attack: that the former Secretary of State isn't healthy enough to serve as president of the United States.

Clinton's woozy moment in hot, humid downtown Manhattan was captured on video. The Democratic nominee is seen looking unsteady, and being helped into a black vehicle by her aides.

"Secretary Clinton attended the September 11th Commemoration Ceremony for just an hour and thirty minutes this morning to pay her respects," her campaign said in a statement. "During the ceremony, she felt overheated so departed to go to her daughter's apartment, and is feeling much better."

And yet hours later her campaign revealed that Clinton had been diagnosed with pneumonia on Friday in a statement from her physician Dr. Lisa R. Bardack. "Secretary Clinton has been experiencing a cough related to allergies. On Friday, during follow up evaluation of her prolonged cough, she was diagnosed with pneumonia. She was put on antibiotics, and advised to rest and modify her schedule. While at this morning's event, she became overheated and dehydrated. I have just examined her and she is now re-hydrated and recovering nicely."

Despite the pneumonia diagnosis, Clinton attended a $100,000-per-ticket fundraiser at the home of prominent attorneys Mary and David Boies in Armonk, New York on Saturday night.

The incident follows intensifying rumors about Clinton's health in recent weeks, fueled largely by the Trump campaign and its surrogates. Just last week, Trump ally Rudy Giuliani stoked those rumors after Clinton had a coughing fit at a campaign event in Cleveland, Ohio. "I have seen eight times online since January in which she's had massive coughing fits in which she couldn't complete her speech," Giuliani told MSNBC's Hardball, but was unable to give specific examples to prove his argument.

The health of both candidates, physical and psychological, have emerged as campaign issues – spurred by their reluctance to release recent medical reports.

Last summer, Clinton released a doctor's note demonstrating her good health, which noted that she suffers from seasonal allergies and an under-active thyroid. In 2012, reportedly while battling a stomach virus, Clinton fainted, hit her head, resulting in a concussion and brain blood clot. A test the following year showed the blood clot had disappeared.

Trump has publicly questioned whether his opponent was suffering from early onset dementia, or the aftermath of a stroke – and has used clips of her forgetting what she was saying mid-sentence or jumbling up her words at events. His claims were fortified by an apparently fraudulent letter from Clinton's doctor which did the rounds online, saying his patient suffers from worsening seizures and dementia.

The letter was "not written by me and are not based on any medical facts," Dr. Bardack wrote in response. "To reiterate what I said in my previous statement, Secretary Clinton is in excellent health and fit to serve as President of the United States."

Trump, 70, has been subjected to similar speculation from the Clinton camp. Clinton and her supporters have described the Republican nominee as "a dangerous demagogue " and "unhinged" who shouldn't be trusted with the country's nuclear codes.

Trump released a brief one-page health report from his doctor dated December 2015. Politifact noted that the language of the letter was "a bit unusual because it sounds like parts could have been dictated by Trump himself." The doctor described Trump's blood pressure as "astonishingly excellent," his stamina as "extraordinary" and his physical examination showing "only positive results." Trump takes baby aspirin and a drug to prevent heart disease – not unusual for a man of his age.

Clinton and Trump aren't the first presidential candidates to battle rumors about their health. Franklin D Roosevelt, who had been paralyzed from the waist down because of polio, carefully choreographed media appearances to avoid being photographed in his wheelchair. John F Kennedy reportedly suffered from Addison's disease, an adrenal gland disorder, but concealed his illness from the public until after he was elected. Thomas F Eagleton gave up the Democratic vice-presidential nomination after he was forced to acknowledge that he had received shock therapy for depression. Since Reagan's nomination in 1980 at age 69 – the oldest candidate to date – the health of presidential candidates has received increased scrutiny.