The question I guess is not what diseases are in decline, but which ones are going up
Type II is insulin resistance. Which is probably caused by our industrial diet and maybe other factors. The rate is increasing dramatically.
Type I is an autoimmune disease. I think it's not increasing. (no idea)
http://diabetes.diabetesjournals.org/content/51/12/3353
The reasons are unknown:
https://www.niddk.nih.gov/health-information/health-topics/D...
I hope you're not a doctor!