Clinton, 68, and Republican rival Donald Trump, 70, both intend to release more medical details in the coming days after the former secretary of state came down with pneumonia, highlighting a focus on their health in the run-up to the Nov. 8 election.
Clinton canceled a trip to California to recuperate. But her campaign drew criticism for waiting until after Sunday's health scare at a New York memorial for the Sept. 11, 2001, attacks to disclose the pneumonia diagnosis, which her doctor made 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.
Questions about the incident reinforced the perception of Clinton as secretive, a view fueled by the revelation that she used a private email server instead of a government one while serving as President Barack Obama's top diplomat.
A federal investigation of that issue said she was "extremely careless" in her handling of classified emails but did not recommend criminal charges.
Though Trump on Monday said health was a campaign issue, he did not attack Clinton over her physical condition. "I just hope she gets well and gets back on the trail," he said in an interview with Fox News.
Clinton said in a tweet that she was feeling fine and getting better. "Like anyone who’s ever been home sick from work, I’m just anxious to get back out there," she said.
Fallon said the Clinton campaign was too focused on making sure the former first lady was well instead of releasing information about her condition during the 90 minutes after the emergence of a video showing her nearly collapsing after leaving the memorial early.
Later on Sunday she emerged from her daughter Chelsea's apartment building and declared she felt "great." Shortly thereafter the campaign announced she had pneumonia.
"The short-term turbulence will be more about the handling of this than the substance," David Axelrod, a former political adviser to Obama, told Reuters.
"Antibiotics can take care of pneumonia," Axelrod tweeted later. "What's the cure for an unhealthy penchant for privacy that repeatedly creates unnecessary problems?"
Responding to Axelrod, Clinton communications director Jennifer Palmieri tweeted that the campaign could have done a better job but said the public knew more about Clinton "than any nominee in history."
Trump had suggested for weeks that Clinton lacks the energy needed to be president. He has raised questions about her stamina, reprising a tactic from the Republican primary campaign when he derided rival Jeb Bush as a "low-energy" candidate.
Clinton flew nearly a million miles as secretary of state and has followed a heavy travel schedule as a presidential candidate. Democrats said her continued attendance at events despite her pneumonia diagnosis proved her toughness.
PRESSURE ON HEALTH
Asked by Fox News about his and Clinton's health, 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."
Trump's campaign has issued a short letter from his doctor saying he was in excellent health with "extraordinary" strength and stamina. But the letter did not mention what medicine Trump might be taking or other details typically included in such disclosures.
Trump, a lover of fast food, is expected to discuss his health regimen in an interview to air on Thursday with celebrity physician Dr. Mehmet Oz.
In July 2015, Clinton released a two-page letter outlining her medical condition that sought to reassure Americans about her health after she fell and suffered a concussion at home in 2012 near the end of her tenure as secretary of state.
Fallon said Clinton never lost consciousness on Sunday and it was not connected to a concussion she suffered in 2012. "There’s no other undisclosed condition. The pneumonia is the extent of it," he said.
In recent weeks, staff at Clinton's Brooklyn headquarters have been hit by an illness, and several aides required medical treatment, a campaign aide said.
Democratic donors on Monday underscored concerns about how Sunday’s episode would play out, with some saying they were not concerned about Clinton’s health but still thought the incident could hurt her with voters.
The health problem was the latest blow for Clinton at a time when Trump has erased most of her lead in national opinion polls and is again competitive in many battleground states where the election is likely to be decided.
Clinton's dismissal of half of Trump's supporters as a "basket of deplorables" of racist, homophobic people on Friday triggered heavy criticism, and she later said she regretted the comment.
Trump has compared the remark to a much-criticized comment by 2012 Republican presidential nominee Mitt Romney that 47 percent of the electorate were dependent on the government.
“I was deeply shocked and alarmed to hear my opponent attack, slander, smear, demean these wonderful, amazing people who are supporting our campaign by the millions,” Trump told a conference of National Guard members in Baltimore.
Trump has made incendiary comments repeatedly as a candidate, including questioning whether a U.S.-born judge could be fair because of his Mexican heritage and criticizing the Muslim parents of an American soldier who died in combat.