Huntington's Disease or sometimes called Huntington's chorea is a severe genetic neurological disorder. The Chorea in the disease consists of the abnormal bodily movements caused by lack of coordination, it also affects a number of mental abilities and some aspects of behavior.
In House M.D, House orders Thirteen to take a test to determine whether or not she has the disease, but she objects because she would prefer to not know whether she has it, the reason for this is because not knowing gives her the ability to summon up the confidence to do things that she wouldn't normally do. In the episode House's Head. Thirteen starts acting strangely and House is suspicious and in Wilson's heart thirteen is forced to take the test and we see her sat in a dark laboratory testing her self. The results for the test come out positive for the disease, thirteen then screws up the results and walks away. It is currently unknown whether thirteen actually looked at the results because the camera doesn't show her looking at them.