It is popular to pathologize many attitudes and aspects of temperament. I hate to add to that rather creepy trend. However, despite my worries, I sometimes wonder if introversion is form of mental illness. If it is, should we treat it? Is it even curable?
In short, my answers are yes, yes, and maybe.
I see introversion as a spectrum disorder like autism or Asperger’s, it can range from mild to severe. In its mildest form, it may be barely noticeable. Similar to other spectrum disorders it may even bring some advantages (e.g., the ability to work or study quietly and patiently). However, in its stronger forms it becomes more disabling. Where physical illness may be treated when there are no symptoms, or those symptoms offer no impediment to life (e.g., high blood pressure), mental illness tends only to be a concern when the symptoms prevent living life to the full. Using this reasoning I propose that introversion, in anything other than mild form, be treated as a debilitating illness.
A person born with asthma that prevents him fully participating in sport is likely to seek treatment; he is not likely to assume that is just the way he is. For some reason the introvert whose disability prevents him fully participating in social life will not seek treatment and assume that is just the way he is. In both case he is correct, that is just the way he is, but in one case he seeks to fix the problem.
Yes, introversion is a form of mental illness. Yes, the introvert should seek to treat it. I am unsure regarding the last question. Is it treatable? Can it be cured? I am uncertain, but feel that a full cure is unlikely. Perhaps the best that the introvert can achieve is some mild easing of the symptoms. Mostly it is about learning to live with his illness.
Nature or Nurture? I suspect it is a large part of nature (genes) but some nurture (introverted nature leading to the development of introverted habits – e.g., joining the chess club instead of the dance class).
More males or females? I have no idea if introversion is more prevalent in one sex over the other. It is, however, for more damaging for males because males must express confidence and dominance.
[Of course, extremely introverted women also have a tough time but they probably have more latitude to be quiet, reserved, and less outward (they can claim they are not "introvert," merely "demure." :)]
What are the chances of protection under the Americans with Disabilities Act? None. Extraverts make the rules. :/