WHITE PLAINS, N.Y. (AP) -

An outbreak of respiratory illness swept through Hillary Clinton's campaign in the weeks before she was diagnosed with pneumonia and fell ill at this year's 9/11 memorial ceremony, campaign aides said Monday.
    
The Democratic presidential candidate abruptly left Sunday's event after feeling "overheated." A video later posted on Twitter showed her staggering and eventually slumping forward before being held up by three people as she was helped into a van.
    
On Sunday, her campaign answered questions about Clinton's health and whereabouts with two short statements, both issued hours after she left the memorial in lower Manhattan. More than 20 hours later, her campaign gave a fuller accounting of the episode, which sparked a wave of bipartisan concern about her health and questions about her political transparency.
    
Clinton's husband, former president Bill Clinton, said in an interview Tuesday with PBS' Charlie Rose that Hillary Clinton is, "doing fine" after a good night's sleep. He added that if there are more health problems that caused her apparent weakness Monday, "then it's a mystery to me and all of her doctors."
    
He said that "on more than one occasion" over the years "the same sort of thing's happened to her when she got severely dehydrated."
    
Clinton spokesman Brian Fallon said the former secretary of state started to feel hot at the event, which took place on a muggy day in New York. There was little shade where Clinton was standing alongside other dignitaries.
    
As family members of 9/11 victims read the names of the nearly 3,000 people killed in the attacks, she decided to leave and get a drink of water. She was wearing a wool suit and had been diagnosed with pneumonia on Friday, a fact her campaign had not made public.
    
In fact, campaign aides said Monday, a number of staff members at her campaign headquarters had been ill in recent weeks, including campaign manager Robby Mook.
    
"I probably came in a few more days than I should have over the last few weeks," he told MSNBC on Monday. "I'm still actually recovering from the cough that I got through that."
    
As Clinton walked to the motorcade, she began to feel dizzy and had to be helped into her van. She never lost consciousness, Fallon said. Shortly after entering the air-conditioned vehicle, he said, she began calling aides and requested to go to her daughter's Manhattan home.
    
At the apartment, she played with her grandchildren, even chasing them around the room, Fallon said. Clinton exited the building on her own, wearing sunglasses and carrying a handbag.
    
She waved to reporters and said, "I'm feeling great. It's a beautiful day in New York." She was then driven to her home in suburban Chappaqua, New York.
    
The public was left in the dark about Clinton's whereabouts for about 90 minutes, sparking widespread speculation about her condition. That was a mistake, Fallon said.
    
"In those 90 minutes that elapsed, we could have gotten more information out more quickly," he said in an interview on MSNBC. "That's on us and we regret that."
    
Clinton's doctor, Lisa R. Barback, later came to her home and conducted an examination. In a statement released by the campaign eight hours after the incident, Bardack said the 68-year-old Clinton "became overheated and dehydrated" at the 9/11 ceremony. By the end of the day, she was "rehydrated and recovering nicely," she said.
    
Campaign aides said it was Clinton's decision to attend weekend fundraisers and the Sunday ceremony, despite Friday's diagnosis. Late Sunday evening, she canceled a California campaign swing scheduled for early this week. Her husband, former President Bill Clinton will headline some of her events, an aide announced late Monday.
    
Hillary Clinton is expected to be back on the campaign trail as soon as Wednesday.
    
"It was her decision to want to press on," Fallon said in an interview on CNN. "But I think after yesterday's incident, everybody was agreed that she should take a couple of days off the trail."