Lou Gehrig's disease. One of my basketball friends, a retired police chief in his late 50s, died of it. He was playing with us at the end of our indoor season in May (rec ball, in the HS gym), was fully incapacitated by November, and died in late January.
Although only a few hundred die of it each year, it seems that everyone either knows a victim or knows someone who knows a victim. One of my classmates in Sunday school who grew up with me in my church succumbed to it, as did the son of the newsletter editor at the museum where I do volunteer work.
My mom died of ALS and it progressed fast. Died 5 months after diagnosis. She was 68.