(If you haven't seen season 3, episode 6 this question contains some small spoilers!)
In season 3, episode 6 we see Michonne getting shot in her leg, shortly afterwards we see her gutting a zombie with her sword resulting in the intestines of the zombie to cover most of Michonne's body.
So why isn't she a zombie now? Clearly blood and other bodily goodies from the zombie cover her wound which should result in a 'active' version of the virus to interact with her bloodstream.
According to http://scifi.stackexchange.com/a/11341/2691
"The active virus seems to cause fever, death, and reanimation. The dormant virus activates upon death, causing reanimation. It's also possible that the dormant virus activates upon contact with an active sample of the virus, which would help account for the rapid onset of symptoms."
Shouldn't she have turned before showing up at the prison?