Diarrhea is an unspecific symptom. Acute diarrhea is usually caused by infectious disease(bacteria, viruses), sudden dietary changed or inappropriate food eating. As feeding related diarrhea, you should stop feeding at first, and give some probiotics which will help digests. Emergency treatment and vet supportive therapy may be required in most severe cases. Chronic diarrhea (more than 3-4 weeks duration) can be caused by several factors like parasites, food allergy, intestines problems with food absorption, liver problems, gastro-intestinal tract tumours. Chronic diarrhea can be more difficult to treat and you need to take your dog to vet for additional investigation.