This is both a German and and English word of the week. Here’s the English Wikipedia entry:
Witzelsucht (German: “joking addiction“) is a set of pure and rare neurological symptoms characterized by a tendency to make puns, or tell inappropriate jokes or pointless stories in socially inappropriate situations. It makes one unable to read sarcasm. A less common symptom is hypersexuality, the tendency to make sexual comments at inappropriate times or situations. Patients do not understand that their behavior is abnormal, therefore are nonresponsive to others’ reactions. This disorder is most commonly seen in patients with frontal lobe damage, particularly right frontal lobe tumors or trauma.
The Wikipedia article goes on forever, even including case studies:
Case #1: A 30-year-old, right-handed man was admitted to the department of neurology for irritability, inappropriate behavior, and morbid hyperphagia with obesity. His inappropriate laughter and persistent pun and joke telling was a sharp contrast to his personality as an intellectual theological scholar, known for his exceptional memory as opposed to his sense of humor. This behavior was generally prompted by environmental stimuli such as physician’s rounds or blood sampling. To the patient, his behavior seemed normal, which explains why he remained nondiscriminating toward his jokes, their context, and their impression on those around him.
I scoured the Internets for clips of people actually suffering from witzelsucht (no capitalization in English), but I found only sarcasm and mockery:
Apparently, people seem to have a problem taking joking addiction … wait for it … seriously.
For an example of an even rarer condition, see this heartbreaking case study, dramatized by English actors:
German Joys 
Terry Jones and Michael Palin are officially the funniest people in the history of the world. Case closed.
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