With Uva's yummies, the process is painless and not all that inconveniencing, save the initial "the HELL is that thing that fell out of my torso" reaction. Basically, after getting a cell medal tossed into ya, the yummy's weak initial form falls out of the host and runs off to start getting shit done, RE: fulfilling host's desire. After a little while, the yummy will upgrade into it's final and stronger form for better havoc wreaking.

..further mutation happens if the host has an epiphany that changes their desire, but this doesn't happen often.

[/teacher hat]