it’s hark’s father who comes up with the zombie virus. it’s mostly an accident – he was just trying to get their small town to pay attention to him and stop shunning him ( even though everything was his fault in the first place, but he’ll gladly forget that fact ). his experiment is a gamble, but he loves experimenting, so he doesn’t consider it an issue. he chooses two test subjects – his own child, because oogie’s an asshole and we all know it, and the son of one of his employees, shock, who’s never cared for her own children anyway.
the virus is in test tubes, and the goal is to see how it plays out on the two of them, but zane ends up kicking the vials out of oogie’s hands, and so instead of hurting the two kids, the contents end up all over oogie. he turns into a zombie just from contact, no drinking needed. it’s a gross transformation and the two need to look away.
one of the town’s residents finds the children beside what seems to be oogie’s corpse, but as she’s freeing them, oogie seems to rise up, eyes glazed, and that’s when everyone realises what’s happened and it’s a mess and everyone just runs for it and warns the town, but no one believes them until they see it for themselves, and some of the people in town don’t make it outside without either dying or turning into a zombie themselves.
zane and hark end up in a group of other kids, their friends ( aka the other halloweentown children ), and they decide to go to the next town for help. as far as they know, the virus is contained to their town alone, and so they take their time, since everyone’s tired. it’s hark that’s been put on watch for the night, and so it’s them that is attacked by the nearest zombie, which alerts the rest of the camp. zane runs out and manages to deflect the blow just in time, but he’s left with a scratch on his arm.
not wanting to turn into a zombie himself, zane begs the others to cut off his arm before the virus can spread, and they do ( albeit hesitantly ). it’s hark that finds a replacement for zane’s arm, one that’s only temporary, and whenever zane gets upset about his lack of a real arm, they’ll sing to him to make him feel better. that goes for the entire camp – hark’s voice helps all of them relax despite the journey ahead of them.