There are multiple causes of diarrhea: it can be caused by dietary intolerance or indiscretion, the presence of intestinal parasites, as well as more serious disturbances. Firstly, it is important to distinguish acute diarrhea from chronic diarrhea : the former lasting only a few days, and the latter having been present for more than ten days. However, the change in the overall state of your companion should be of greater concern than the amount of time it is afflicted with diarrhea.
