When 88 Colorado River rafters - including 22 experienced river guides - came down with stomach flu-like symptoms this June, they sounded the alarm. "That's a lot of cases in a relatively short period of time," says Marlene Gaither, a Coconimo County health specialist. "When it's 100 degrees on the river and you vomit and have diarrhea, you can dehydrate pretty easily." In response, the Park Service has warned anyone using the Colorado River this summer to avoid drinking untreated water. Gaither says the outbreak may have been caused by a virus carried in human waste. Although regulations require all Grand Canyon river runners to use portable toilets, there are no guarantees everybody abides, she says. In mid-August, when the number of cases had increased to 108, state health officials set up a small clinic on a beach deep inside Grand Canyon. More than 400 people coming down the river were surveyed during three days, and five who had experienced symptoms agreed to give blood. The samples have been forwarded to the Centers for Disease Copntrol and Prevention in Atlanta, Ga., for testing. Meanwhile, the number of cases has dropped off dramatically. Says Bill Gloekner of Arizona River Runners, Inc. in Flagstaff, "Luckily, it's getting back to normal."