It depends on how it starts. If you are lucky enough to deal with the initial carrier before it have infected anyone and dispose of it successfully, then that's the end of it. But situations can escalate quickly into a class one or two outbreak, especially since (based upon the Solanum Virus model) it can take nearly 24 hours from initial infection to reanimation, so you may find zombies continue to emerge for some time after the event, perhaps regaining a foothold. But it always starts with a single creature, be it your basic class one or apocalyptic class 4 incident, since the natural origin and residence of the virus is unknown.
Then again,
this should give you all a chuckle.