Diarrhea causes loose or frequent bowel movements. In pets this is often the result of them eating something they shouldn't or an intestinal virus. The number one cause of diarrhea in pets is dietary indiscretion (eating something inappropriate such as access to people food, garbage, poisons, a sudden change in diet or a stressful event/environmental change). Other common causes of diarrhea can be something more serious such as infections from viruses or worms, drugs, and underlying medical problems. If the diarrhea is severe take your pet to the vet as soon as possible.