How does poison work in FATE?
Secret, previously administered poison (say, in a drinking glass) is an aspect that, like most investigation aspects, doesn't expire 'next turn' but will sit and wait.
Extremely fast poison can just be thought of as damage on the weapon, it does damage when it hits. (Say, acid weapons or most of the fae nasties that Harry runs into.)
What if someone gets hit with a poisoned weapon that would last for at least a few rounds, preferably how to do it over a few minutes/hours?