
1: "90% of diseases" is not the same thing as "90% of patients".

   In a world with one curable disease that strikes 100 people, and nine
   incurable diseases which strikes one person each, medical science will cure
   91% of the patients and report that 90% of diseases have no therapy.

2: A disease would be counted among the 90% untreatable if nothing better than
   a placebo were known.  Of course MDs are ethically bound to not knowingly
   dispense placebos...