Probably around ten percent of the global population is dealing with mental illness. Probably a tenth of those are dealing with a disabling mental illness, and probably a tenth of those are dealing with a severe mental illness. The last is the type where you end up in the state institution, or in assisted living. The first would be characterized by a condition that requires one or two pills. If your illness is disabling you’re considered “seriously mentally ill”. That’s characterized by hospital stays, med cocktails, etc.
Diagnoses are becoming less common. While the autism spectrum is still distinct, there is a ton of overlap between schizophrenia, schizoaffective disorder, bipolar disorder, and borderline personality disorder. ADHD, also, has almost everything else as a possible co-morbid condition. “Mental illness” can be thought of as a condition unto itself.
The main external symptom of mental illness is aberrant behavior. The main internal symptoms are anger, fear, hopelessness, suspicion, fatigue, mental confusion, and an inability to articulate thoughts and emotions correctly.
Mentally ill people tend to stick close to home, because they often need someone to act as a caretaker. They universally deal with chronic fatigue, and many battle with “self neglect”, i.e. keeping up with simple activities of daily living, such as cleaning, bathing, feeding themselves, etc.
Mental illness is incurable, and a birth defect. An imbalance of neurochemicals. The best means of showing understanding is in acceptance, rather than attempting to fix or change the situation, or dismiss it.