Depends which psychiatric illness...
A symptom is a characteristic of illness, to which the antonym would be normality.
Diabetes can be a dormant illness, meaning it often does not show symtoms until later down the road, or they can be very subtle. One symptom of is a highly unquenchable thirst and frequent urination.
Yes
Anything can be a symptom of something, contact a doctor if you're seriously concerned about it.
Hepatitis A
No, may be an Weak stomach or "the runs"
Not eating is a symptom of mental illness. It can also be a symptom of other medical problems. Since it is a symptom, it is extremely difficult to determine if it is a cause.
I would say its a symptom of something. You should get an appointment with the vet.
Bean-eating addiction.
Yes, Diarrhea is a symptom of a disease or illness
Cachexia