Topic/disease cleanup

This part was hard to read, and had some duplicated code for handling corprus checks.  I think this should be easier to follow later.

I just saw the repetition of constant math on each set of resist, and pushed it down to the x calculation.  Once it was there, the * 100 followed by * 0.01 looked a little silly...

But as scrawl pointed out below, consistency with the wiki notes is probably more important than eliminating all the repetition.  The update shifts the constant math back.