Not saying the Z word, eh? Or are you talking about butter?
I think the virus will spread relatively fast, especially if it starts in a crowded place. If the victim is brought to a hospital, then even worse (unless they could cure it or whatnot).
Panic will start and create a huge rush once more find out what's going on. Maybe if an outbreak started and it wasn't covered up, then the panic would spread. It'll start slowly, but as the numbers of THEM increase, it'll spread fast. The more populated a place is, the faster. I would hope that the response against the things would be swift and efficient, but that won't always happen. People will start fleeing and infected will pass it on to other places, all that jazz. No quarantine is 100% effective. But unlike your typical movie character, we would at least have some idea of how to deal with the threat.
If you're talking about butter, then it will depend on what knife you use and how fast you are.