The Baron's 'disease' isn't adressed in the original novels (beyond mentioning that he's very fat) but it was in the prequel novel co-written by his son. Note that the specific disease isn't mentioned, nor does it fit the description of any contemporary plague. Clearly it's something either very rare or something that's arisen in the human population in the next 10-15000 years.
"That was when Mohiam selected the particular disease, an oh-so-gradual vengeance, a neurological disorder that would destroy his beautiful body. The Baron’s physique obviously brought him much pleasure, was a source of great pride. She could have infected him with any number of fatal, suppurating plagues— but this affliction would be a deeper blow to him, much slower in its course. She would make the Baron face his own appearance every day as he grew fatter and weaker. His muscles would degenerate, his metabolism would go haywire. In a few years, he wouldn’t even be able to walk by himself"
Dune: House Atreides
What's most find interesting is that this idea (e.g. that the Baron's condition was given to him by the BG) wasn't in the original Dune series of books, but was included in the prequels co-written by Herbert's son, which means that either the author of the encyclopaedia had inside knowledge of genesis of the Baron's condition or that Brian Herbert took the idea from the encyclopaedia.