Erdheim-Chester disease

Erdheim-Chester Disease is a type of histiocytosis. Although it's quite rare, it tends to affect people in middle age, with the average age of the onset of symptoms being 53. Misshapen and fat filled cells invade the bone marrow and the long bones, causing pain and other complications. It's difficult and expensive to treat, and isn't curable.
Erdheim-Chester disease at Wikipedia
As a fictional disease depicted in All In, Erdheim-Chester Disease is a very rare disease with fewer than 300 reported cases. It first attacks the digestive tract, causing bloody diarrhea, but during its early stages cannot be confirmed by a biopsy of the large intestine. It next causes neurological damage, leading to ataxia. Next, the kidney and liver fail. Death generally occurs within 24 hours of the onset of the first symptoms. However, if diagnosed quickly, the disease can be treated and reversed.