The cycle for some parasites is 5 or 6 stages. In relative size changes btween each cycle can be a small as a mouse and tall as the statue of liberty.

I was interested in the ones that cause subtle andspecific behavior changes in the central nervous system. Its amazing.
Fatal infections are like a weapon system and do not interest me.

In my first draft I used dramaticized historic examples of these micro monsters leaving mysterious clues and changing human history.