Well, that is hard for me to answer because I view few mental health issues as 'illness'.
To me, BPD is the result of missing out on early experiences of attachment, or experiencing trauma. Something needed in development was somehow missed out on for some reason.
The term BPD to me is a way to describe the difficulties we have in relationships, work, and also the intense emotions and intense moods we experience... because of our perceptions formed early in life.

That's why I like the term 'disorder' rather than 'illness'. Just me