Pneumonia bug that struck Hillary also sickened several members of her staff

“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.