Define "activated"?

There is something I am not quite getting about your statting. Maybe I don't own the right splat-books. What is a "Legendary Disease"? Why does it seem to have two forms when discussing size, and how does it switch between them and/or between "evolutionist" and "sneaker's mindfire"?

The following phrase seems grammatically garbled, and may be contributing to the confusion:
Any (Ex) or (Su) or mutation or feat the source creature attains when on the same plane as the Phage it also gets with the same optional variables,