Mental illness is a psychological illness.

Physical illnesses can always be determined through some type/s of test/s: like x-rays, blood test, mri's, eeg's or CT scans...and so forth.

Instead, psychological illnesses aren't crisp and clear cut cases. There needs to be patterns in behavior and/or outlook on ___, certain answers to a variety of tests, noticeable differences in tone of voice, eye contact, posture, dressing style, cleanliness, and so on.

Sometimes, untreated medical conditions can bring out mental illnesses. A wide variety of treated medical conditions often do cause depression and anxiety to otherwise "healthy" people. Brain physical illnesses typically cause mental health illness symptoms as well. That is why proper testing needs to be done. To rule out possible physical illnesses in the brain.

That's my answer to your Q's.... IMO, it doesn't get more simple than that. But, then again, maybe I'm looking too deeply. {I have a tendency to do that every now & then.}