In the great outdoors, the only word that strikes fear in a backpacker's heart faster than "Grizzly!" is "crypto" (short for Cryptosporidium). Crypto is a microscopic, waterborne parasite that the federal Centers for Disease Control has tagged a major health risk to those who spend time in the woods.
The reason: It's very easily transmitted, usually in drinking water, but just as easily by an infected hiking partner's dirty hands.
The hearty critter sets up housekeeping in the intestines of everything from your unhygienic tentmate to a cow upstream. It travels via feces and, thanks to its tough shell, can survive for long periods without a host, even in very cold water. (Warning: Frozen streams should not be considered crypto-free!)
Swallow a few crypto cooties and 2 to 10 days later, you may develop stomach cramps, flatulence, loose stools (or full-blown diarrhea), a mild fever, and general unhappiness. Symptoms can last from a few days to a few weeks. It's common for crypto victims to get better, then worsen, then finally get better for good. Cryptosporidiosis usually is a relatively mild illness in otherwise healthy people. But unlike giardiasis, another waterborne parasitic infection that's often treated with antibiotics, there is no medication proven to help you overcome cryptosporidiosis. Your body eventually just has to rid itself of the parasite.