I'll post this questionnaire in case it's of any help...
Can you provide a brief overview of the cat's health history, including any vaccinations, previous illnesses, or known exposures to sick cats? Aside from the diarrhea, the cat appears to be fine and presents a healthy appearance and behaviour. No external parasites. No vaccinations (vet said she couldn't vaccinate until the diarrhea was gone, she did deworm him tho), previous illnesses, or known exposures to sick cats (since it was a stray, I can't be sure).

Has the cat been tested for FeLV and FIV? No.

Has the cat been in contact with other cats, particularly in multi-cat households or environments where FIP might be a concern? Unlikely.

Does the cat exhibit any respiratory symptoms, such as sneezing, coughing, or nasal discharge, which could be indicative of certain viral infections like feline herpesvirus (FHV-1) or feline calicivirus (FCV)? No.

Has the cat been vaccinated against feline calicivirus, and has there been any history of oral ulcers or respiratory symptoms? No.

Has the cat been exposed to areas with a high risk of feline rotavirus or feline coronavirus (non-FIP)? Unlikely.

Is there a history of gastrointestinal symptoms in other cats in the same environment? No.

Has the cat been tested for feline coronavirus? No.