This a hard one because I've feared so many things. I guess MS fears and ALS. Doesn't mean I don't have an neurological fears, as my mom died of a disease like that, but I've never consciously worried about MS or ALS. Not sure why, because they can very devastating diseases. Honestly I've never worried about having liver disease either. But I have it now (autoimmune) so that's a weird one.
Cancer and heart were my biggies, especially ovarian and pancreatic, and heart attacks.