Because our lives are so intertwined with the habits of our pets, we tend to know right away if they’re having gastrointestinal problems. Vomiting and diarrhea are pretty obvious signs that something isn’t quite right with your pet. Allergies, illness, cancer, infections, drugs, parasites, plants, and poisons can all cause vomiting. Diarrhea is one of the most common illnesses of dogs and cats, and is especially common in puppies and kittens. Depending on the cause of diarrhea, you may need to change diet or administer medication, or both.