This is a very common disease and I'm surprised there isn't a board for it here; most other large communities have one.
Dysautonomia/Autonomic dysfunction is basically a malfunction in the autonomic nervous system and encompasses alot of different conditions. It affects the majority of our "inner" workings and most things that our bodies do automatically.
It can include postural orthostatic tachycardia syndrome (POTS), Orthostatic Intolerance (OI), Orthostatic Hypotenstion (OH), inappropriate sinus tachycardia (IST), Neurocardiogenic Syncope (NCS), mitral valve prolapse dysautonomia (MVPD), pure autonomic failure (PAF), vasovagal syncope, neurally mediated hypotension (NMH), autonomic instability and a number of other conditions.
The cause of Dysautonomia is not quite understood and very difficult to diagnose in many patients.