Could you tell me what are some common illness causing parasites/microorganisms found in creek water?

Giardia lamblia: Giardia infection is a waterborne infection and is caused by parasites found in streams and lakes. The giardia parasite is a common intestinal parasite. The most common way to become infected with giardia is after swallowing contaminated water. To help diagnose giardiasis, your doctor is likely to test a sample of your stool. Doctors usually treat giardiasis with Metronidazole /flagyl.