Dead Or Alive

Having the medical knowledge of a house brick, i always assumed like most people, that a vaccine contained a weakened, harmless version of whatever it was you was trying to be inoculated against so the body would generate the required anti-bodies to overwhelm the weak disease and would then stand guard ready in your body in case the full blown thing happened along.    
Rather kindly the scientist didn't slap his forehead and mumble under is breath about being surrounded by idiots but patiently explained that it is a harmless version of the disease, actually completely harmless because it is dead and one of the laws of medicine is dead things don't come alive.
Putting aside questions about how could he then explain the coming Zombie Apocalypse, i just looked confused instead so he went on to say that the body will recognise that this new thing shouldn't be there and is attacking the body, albeit dead, so it sends it anti-bodies to attack and nullify it and then makes the decision that the anti-bodies had better hang around a while in case there is another attack and over time, as the threat of another attack resides, the anti-bodies go off to do whatever unused anti-bodies do.
Nodding as if i understood what the hell he was talking about, he then said that actually they do sometimes use weakened versions of whatever it was they were inoculating against also for the same reaction which i realised as i was driving back to work meant that i was kinda right all along, go me.
Makes me glad i never asked about the Zombie Apocalypse after all.