Hillary Clinton is scheduled to return to the campaign trail on Thursday after taking a few days off to recuperate from pneumonia, a campaign aide said on Tuesday.

Clinton "has spent the day catching up on reading briefings, making calls" and watching on television President Barrack Obama's speech at a campaign appearance for the former secretary of state in Philadelphia, spokesman Nick Merrill said.

"We will resume campaign travel on Thursday, more details to come," he added.

Clinton had said Monday that she is "feeling fine" and hoped to get back on the trail soon.

"Thanks to everyone who’s reached out with well wishes! I’m feeling fine and getting better," Clinton said on Twitter, adding in a second tweet, "Like anyone who’s ever been home sick from work, I’m just anxious to get back out there. See you on the trail soon."

In an interview with CNN Monday, Clinton said she would resume campaigning in a couple of days, after she had canceled a trip to California. Clinton added that she initially didn't think her diagnosis was "that big a deal."

Her campaign acknowledged on Monday it may have been too slow disclosing her pneumonia diagnosis, which didn't become known until Sunday when Clinton nearly fainted at the Sept. 11 memorial service at Ground Zero. She was diagnosed with the lung infection on Friday.

"I think that in retrospect, we could have handled it better in terms of providing more information more quickly," Clinton spokesman Brian Fallon told MSNBC.

Clinton left the memorial service early after becoming “overheated” during the ceremony, according to her campaign spokesman. Clinton's doctor later released a statement explaining the Democratic candidate had been diagnosed with pneumonia on Friday.

A video posted to Twitter by user @zgazda66 appeared to show Clinton stagger and struggle to get into a black van as her staff and security assisted her. Clinton attended the ceremony for an hour and a half before leaving to head to her daughter Chelsea Clinton's Manhattan apartment, according to her spokesman.

She “is feeling much better,” the spokesman said Sunday. Clinton was spotted leaving her daughter's apartment shortly before noon.

"I'm feeling great, I'm feeling great," she said. "It's a beautiful day in New York," she added, while smiling and waving.

The statement released by Clinton's doctor, Lisa R. Bardack, said she became "overheated and dehydrated" at the memorial.

"I have just examined her and she is now re-hydrated and recovering nicely," the statement said. It also said Clinton had been put on antibiotics and was advised to rest and modify her schedule before attending Sunday's memorial service.

Former President Bill Clinton said that his wife has had health episodes in the past similar to Sunday's near collapse, but has worked through them.

"Rarely, on more than one occasion, over the last many, many years, the same sort of thing's happened to her when she got severely dehydrated, and she's worked like a demon, as you know, as secretary of state, as a senator, and in the year since," Bill Clinton said in an interview with "CBS This Morning" co-host Charlie Rose, an excerpt of which aired on Monday night.

Republican presidential candidate Donald Trump said on Monday that health is an issue in the campaign.

"I just hope she gets well and gets back on the trail, and we'll be seeing her at the debate," Trump said in a phone interview with Fox News.

Asked if the health of the candidates was an issue, Trump said: "I think it's an issue. In fact ... this last week I took a physical and ... when the numbers come in I'll be releasing very, very specific numbers."

By Monday afternoon, a spokesman said Clinton will also release additional medical information.

Past presidential candidates have released much more detailed information about their health than either Trump or Clinton.

John McCain, the failed 2008 Republican presidential nominee, allowed reporters to see 1,173 pages of medical records after concerns were raised about a cancer scare.

Clinton has been in the news before for serious health issues.

In December 2012, she suffered a concussion and shortly afterward developed a blood clot.

In a letter released by her doctor in July 2015, Clinton was described as being in "excellent health" and "fit to serve" in the White House. It noted that her current medical conditions include hyperthyroidism and seasonal pollen allergies.