Basically, prairie madness and neurasthenia occurred when women literally became bored to death. Having nothing to do for great stretches of time, women became sluggish and unmotivated, and the brilliant doctors of the day prescribed wonderful treatments like staying in bed for three months at a time, opium laced tonics, and removal from all intellectual excitement. In retrospect, it doesn't sound like such a bad deal, does it?
For a wonderful description of prairie madness in action, take a look at Laura Ingalls Wilder's These Happy Golden Years, in which Laura (is it a coincidence that Waylaid's heroine has the same name? I think not) is sent to live with crazy Mrs. Brewster, who sits in a chair all day and sulks, and by night stalks around the house with a butcher knife. Fun!