Look at Dr. Hulda Clark 's Bowel Program. One of the herbs, turmeric, is very effective against some of the enteric pathogenic bacteria. Cayenne may also help.
Some other illness such as diabetes and gluten sensitivity (Celiac Disease)often are involved in diarrheas. You may need to rule out food allergies if this problem persists.
If you have Giardia the usual stool exams as mentioned in another post will probably find it but to id amoeba and other protozoa it may take special lab tests and exam by a gastroenterologist.