You could easily add a Talent for this just like the Resistance Talent currently works. For each 1 or 2 points the character could get a +1 to their CON Roll to not catch the disease. More powerful diseases would then purchase minuses to the CON Roll. So a cold might be -0, the flu -1, the pox -5, etc. You would just list out all the diseases and what their minuses to the CON Roll would be.