It must be a pretty nasty one, because the optics of a health-related event cancellation are pretty bad for the 71-year-old (as demonstrated by Halperin's instant home-page hyping). It's extra-curious because candidates power their way through sickness all the time, and McCain probably would have been better off doing a sub-par town hall than getting a headline like this.
Update: Something strange is happening here.
--Michael Crowley