milk, human food and some other stuff
There are multiple reasons - parasite infestataion, bacterial infection or overgrowth, inflammatory bowel disease, etc. Persians are also somewhat high maintenance and are quite sensitive to their...
the way humans do...if it eats something that does not agree with it
If your cat has diarrhea, give him Feline Gastralox. It is the best over-the-counter medicine for cats with diarrhea. You can find it at Amazon.com.
Take him to the vet. It could be illness or food allergy.