Korsakoff's Syndrome is a vitamin deficiency disease caused by a lack of thiamine (vitamin B1). The body requires only small amounts of this vitamin, so the syndrome is usually only found in people who are severely malnourished, or alcoholics who have almost no other food source. As House points out, this means that patients will almost certainly weigh less than 100 pounds or will have the red nose characteristic of the breakage of capillaries in alcoholics.
The vitamin deficiency eventually causes brain damage, leading to severe amnesia and the unique sign that defines the diseases - confabulation. Patients are not aware they have no memories and make up memories from environmental clues in response to questions.
Treatment with thiamine will quickly stop the progress of the disease, but usually does not reverse the neurological symptoms. This usually requires a course of drug therapy which improves brain function, but may not restore it entirely.
House has pointed out that this disease embodies his credo everybody lies. He advises students to treat all patients as if they have Korsakoff's.