Alien-hand syndrome, no it isn't a disease you catch from close encounters of the third kind. It's actually is a rare condition that affects patients of epilepsy causing them to have little or no control over a particular limb(s). This condition occurs because the two hemispheres of the brain are not communicating properly. The left and right hemispheres usually share information simultaneously but when the line of communication of severed, each side will think completely independent of each other in certain situations. For example, a women will choose her outfit for the evening and all of sudden her alien limb will put away the outfit.
Surprisingly Alien-hand syndrome is from deliberate damage to the brain. Epileptic patients undergo an intensive surgery as a last resort effort to relieve painful, uncontrollable seizures. The thick fibers called the corpus callosum connecting the left and right side of the brain are cut thus, breaking off communication between left in right hemispheres. The result is a body controlled by almost two different brains. Alien -hand syndrome usually subsides when the brain finally adapts to its new physiology but in some cases one side will never submit to the dominating hemisphere.
Below: Karen Byrne of New Jersey has suffered with Alien-hand syndrome for over 18 years. She is one of the rare cases where her right side refuses to be dominated by her left side. She is now receiving drug treatment that slightly relieves her condition. In this video she is fresh out of surgery already afflicted with syndrome of her left hand uncontrollably slapping her face. (skip to 1:10 for actual footage of Karen)