Is an unhealthy interest in someone almost to the point of obsession a sign of mental illness?

Stalkers are the only kind of people that have an unhealthy interest in someone. A sho 'nuff sign of a mental illness.

Not necessarily.  Today people want to say everything is a mental illness..................they watch too much daytime useless TV.  Any time something becomes an obsession a person has gone into dark water meaning they have lost perspective.  Most of us can see that and regain control.  Some don't and they can end up needing professional help.  That still may not be a mental illness.  Every out of whack display of emotion is not mental illness.

My entirely unprofessional opinion would be all obsession which disrupt one's general life may be a sign of mental aberration.   The more disruptive an obsession is, the degree to which it may adversely effect the one who holds it and the object of that desire ( if a person) the more serious the matter.

yes, and thats ocd

Dear O-uknow,
I would say it almost doesn't matter whether the obsession is officially mental illness or not, it confines and cripples someone's life, and somehow need to find a way to move on.

What do you mean by "unhealthy"? What do you mean by "obsession"? What do you mean by "mental illness"?

In my opinion, yes. 

With the use of the word "unhealthy" in your question, the answer is an automatic affirmative.