Hillary Clinton stumbled getting into a van after a Sept. 11 memorial service in New York City on Sunday

By Sandra Sobieraj Westfall Stephanie Petit
September 12, 2016 08:20 AM

The illness that sickened Hillary Clinton with pneumonia and caused her to have to be escorted away from September 11 ceremonies in New York on Sunday also struck down several members of her campaign team at Clinton headquarters in Brooklyn, PEOPLE has learned.

“Everyone’s been sick,” a campaign source tells PEOPLE.

At the end of August, two weeks before Clinton was diagnosed with pneumonia, a debilitating bug was making the rounds among staff at her headquarters and campaign aides who travel with Clinton, a source tells PEOPLE.

At least half a dozen senior staff were felled, including campaign manager Robby Mook. Two top advisers even needed emergency medical treatment, the source says. One top adviser diagnosed at a Brooklyn urgent-care center with a respiratory infection was being treated with antibiotics in the days before Clinton’s diagnosis. Another top adviser was taken by ambulance to the ER after collapsing from what turned out to be severe dehydration, the source said.

Hillary Clinton was filmed stumbling as she was escorted into an SUV on Sunday

Clinton has canceled plans to visit California on Monday and Tuesday so that she can rest and recover at her home in Chappaqua, New York, according to her campaign.

The Clinton campaign initially blamed the 68-year-old Democratic nominee’s health issue at the 9/11 memorial on being overheated. Hours later, her doctor released a statement revealing that she was diagnosed with pneumonia on Friday.

Video taken at the event showed Clinton stumble as she is helped into an SUV by staffers near the World Trade Center on Sunday.

Hillary Clinton at a ceremony at Ground Zero on Sept. 11, 2016
Albin Lohr-Jones/Getty

Later in the day, Clinton emerged from daughter Chelsea’s apartment and was seen smiling and waving to the crowd.

A source who was with the candidate on Sunday tells PEOPLE, “She is fine now. She had been standing for a while in the heat. After she cooled off at Chelsea’s, she made a point to take a walk outside Chelsea’s apartment building to prove she was OK.”

Presidential campaigns are always grueling – interminable work days, hours spent on and off of planes, the stress of the high stakes – for the candidate and staff and traveling press corps alike.

That’s why one veteran of the campaign trail, Nicolle Wallace, former communications adviser to President George W. Bush, says Sunday’s belated disclosure of her pneumonia diagnosis – at a time when polls show that Clinton’s trustworthiness is seriously questioned by a substantial chunk of voters – is more a political issue than a real medical one.

This shows, Wallace said on NBC’s Today, that “the Clintons can’t come clean about anything, including cold and flu season.”