There is a "natural history" of evaluation and assessment. It happens in a context and follows certain procedures. Often involves observation and some history taking. Generally the diagnostic manual (I do not have DSM 5 yet) gives a list with options under each part of the list. So, say a list with A, B, C, and D. For schizophrenia is may give items under A as premorbid (before illness or active phase), items under B as active phase symptoms, and items under C as perhaps residual phase symptoms. D may give special symptoms for child onset schizophrenia. This is just an idea and it may not look like this in the actual DSM IV-TR.
For the symptoms under A there may be a list of seven symptoms and it gets checked as true if the person has four of these. Likewise for the other lists.
So, schizophrenia can be an illness exemplified by a fairly wide diversity of persons. Sometimes the symptoms can be more or sometimes less severe.
Generally to have a severe mental illness the symptoms must severely affect day-to-day living. In "The Skilled Helper" by Egan which is the Bible for the helping professions, at least IIRC 4th edition, severity is given by the following formula:
S = DUF
Or, severity equals distress x uncontrollability / unpredictability x frequency
Other illnesses have other sets of lists among which you choose which ones apply and if it meets the threshold to make a diagnosis decision.
There are also "differential diagnoses" where sometimes two illnesses do not exist together and one must decide which one it is. Sometimes two or more illnesses do exist together. Sometimes both illnesses together becomes a distinct illness.
Putting "mental disease" on a thread title is probably going to attract people who may have a mental illness diagnosis. Even those who might fear they have a possible undiagnosed illness. So this thread is probably no clue as to what happens on other threads.
Some people may come to this website to frequent the mental health area with the website and do not go elsewhere. Some people may be seeking counsel or prayer. Note also that a hospital has sick people and health professionals in it and concerned family and friends. This website may be similar to a hospital at times.
The discussion of "normal" and "abnormal" could take a thread on its own. If someone asks me to guess their IQ and I have no additional information, then the best guess is the mean or average IQ which is around 100 and it drops off dramatically as you go down below 60 or above 140. Simple definition of "normal" is thus "average."
Besides IQ, other traits can be normally distributed. Psychosis, Emotional states and traits, sexuality, and motivation might be normally distributed too.
Disability is a very broad category including physical disabilities, learning disabilities, psychiatric disorders, sensory disabilities, dementias, developmental disabilities, neurological impairments, visible and invisible disabilities, and there surely must be others that I am not recalling. Addictions, personality disorders, or whatever may be special too. Society is sometimes expected to make "accommodations" for the disabled and sometimes not. We do what is reasonable. The whole legal system is based upon the "reasonable person" (is this a legal fiction?). Or, is Jesus "the" reasonable person?
I imagine that some people looking at posts can do a mini mental status exam while reading posts. This is a face-to-face assessment. It has about 10 areas which are assessed such as memory, orientation, grooming, sensorium, etc. Sometimes, as a psychiatrically disabled person myself I do an internal or personal monitoring MMSE just to see how I would do if I were assessed.
I can also, if I wish, do a preview of a post before posting and do a personal self-assessment of how "abnormal" the post looks. If somebody were "normal" on all variables, then this person would be highly abnormal. Or faking.
When people are being assessed, they can fake good, fake bad, fake average, or fake random, and, there are ways of detecting all of these but these ways take questions and time.