Sierra hikers may be tempted to drink from cool mountain streams, but few such streams are free of pollution. Untreated water carries a risk for Giardia, a parasitic organism that causes chronic diarrhea. Jeff Tanji, sports medicine physician at UC Davis School of Medicine and Medical Center, advises Sierra campers and hikers to carry their own water or take chemical tablets or mechanical filter pumps along on camping trips. Both methods of treating water are inexpensive and readily available at hiking or camping supply stores. "Giardia has to be treated with medication prescribed by a physician," Tanji says. "Why risk ruining your outing?"