Cryptosporidium is a parasite similar to giardia that can cause cramping, illness and diarrhea when ingested by drinking untreated water. It can be stopped with a water filter. Cryptosporidium has a hard outer shell that makes it difficult to kill; it will live for long periods of time once it enters the body and it is resistant to chlorine.
Although cryptosporidium and other waterborne parasites are a genuine hazard for hikers, especially those who drink unfiltered water,the primary cause for gastrointestinal discomfort on the trail has been blamed on poor hygiene.
Basic discipline in cleanliness, such as hand washing and filtering your water will help to prevent illness while enjoying the trails.