Multiple Personalities

When we watched the video about multiple personality disorders I found it hard to believe.  I don't understand how someone can have that many personalities and not understand how they are acting.  Although it's hard for me to believe I still find it interesting.  The thing that makes it so hard for me to believe is how she can act so mature one minute and then change to a 5 year old girl the next minute.  In the video it seemed like it is very dagerous if you have this disorder.  I dont see how her husband could stay with her when she has that many personalities and puts their family in danger.  I am also interested in learning about how to treat this disorder.  It did't cover how to do that in the video.