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 by ambulance after collapsing from what turned out to be severe dehydration, the source said.















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.





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."

Pneumonia Bug That Struck Hillary Clinton also Seriously Sickened Several Members of Her Staff| 2016 Presidential Elections, Battling Illnesses, Hillary Rodham Clinton