To have an illness, one must have a diagnosis.
My point was a third class, or second class, or first class medical exam does not test for mental illness. Absent a disclosed condition, if a Dr. suspects something he can defer issuance of the medical. Most Dr.'s are not really equipped to make that kind diagnosis during a routine physical exam (apparenty Dr.'s should consult with pilots on a chat board who can make that diagnosis - insert eye roll here). I doubt many medicals are deferred for this reason.