Jen Theisen was at the 10-mile mark of a 16-mile July hike at Afton State Park southeast of St. Paul when it happened. "My lower legs felt a little prickly," the 46-year-old Crystal, Minn., woman recalled. "And then I knew it was starting." She knew, also, what would come next: a reddish rash spreading across her lower legs, turning bright red by the following day. An uncomfortable, if not exactly painful, itching would last for a couple of days.