A lot. There are a couple of diffrent views on it.
BPD can not be treated by mediciene, BUT medicene can help. There is no cure all drug for it at this time.
You can have this disorder, and other things as well, or just have bpd.
Most commonly people who are diagnosised with BPD have PTSD (like me).
It is mostly the effects of what you weren't given enough as a child, or was given too much of. Not enough protection, statiblity, love, nurturance (and other things), or maybe too much protection. There are basic core childhood needs that every child needs as they are growing up. If they don't get it, then they may grow up and end up with this disorder.
It is curable. (there is some controversy over that though).