It’s a campaign tradition—though we aren’t talking negative ads or push polling. Every four years, a nasty cold virus—sometimes even the flu—makes its way through the campaign bus, afflicting staffers, reporters and occasionally the candidate. The latest victim of the campaign cold: passengers on the Straight Talk Express and the “McCain Train,” the massive press bus trailing the Arizona senator as he barnstorms through South Carolina in the final days before Saturday’s GOP primary. (Yes, the McCain Train happens to be a bus. Don’t ask.)

The first symptoms hit late last week, after the New Hampshire primary, when a TV reporter lost her voice and reported flu-like symptoms on the McCain’s flight to South Carolina. At the same time, a separate strain began making its way through McCain’s top staffers, a virus that was no doubt helped along by a grueling travel schedule and significant weather changes. (In Friday, McCain campaigned along the South Carolina coast, where it was almost 80 degrees. By Saturday, he was in snowy Michigan, where the wind chill was dipping below zero.) Arriving back in South Carolina on Tuesday, almost a quarter of the staffers and reporters traveling with McCain were sniffling, coughing and sneezing—a number that further increased Wednesday, when the McCain buses began to resemble something of a rolling pharmaceutical road show, starring nearly every variety of over the counter cold medicine on the market. (This reporter currently is packing two bags of Ricola cough drops and a box of Tylenol Cold Severe, thanks to a generous friend from National Public Radio with a car.)

Even McCain hasn’t escaped. On Wednesday, the senator woke up with a raspy voice and congestion. “It’s the second cold I’ve caught in two months, and I don’t generally don’t ever get colds,” McCain groused to reporters on his bus. But there’s no time for rest. Two days before the vote, McCain is on top of the polls here—leading Mike Huckabee by 29 percent to 22 percent, according to a Reuters/C-Span/Zogby poll this week of likely GOP voters. Yet McCain isn’t chancing anything. Coming off a 15 hour day Wednesday, he’s got three rallies/town halls scheduled for Thursday and another four on Friday. Luckily, McCain has a physician on board, just in case: Sen. Tom Coburn from Oklahoma, a part time doctor who endorsed McCain on Wednesday.