Zombies in Love by Nora Fleischer

That’s how I knew I was infected.  I’d get a papercut, or a little burn, and the next day it would be totally gone.”  She looked up at him, then looked away quickly.  “Maybe it was that way for you, too.”
    “No,” he said.  Then he shook his head.  “That’s not right.  I had knee surgery.  They told me it would take two months to recover.  But it only took one."  And if that was the case, he’d been infected for at least two years before he died.
    How had that happened?   And what, exactly, did she mean by infected ?  He'd slipped into her terminology unconsciously, and maybe it was just that he wasn't a scientifically minded person, but he'd never thought of what had happened to him as a disease, but rather some kind of well-deserved supernatural curse.  The mechanism was irrelevant.  He already knew why he had come back.  It was simple.  He was a monster because he'd behaved like a monster to Sam.
    Did Sarah know something he didn't?
    Sarah set her tea down. “When it happened, when you became a zombie, did it happen all of a sudden?  Or was it, you know, bit by bit?  How did you know it was happening?”
    He stared at her, unsmiling.  “First you tell me how this started.  Then I tell you what I know.”
    She fidgeted in her chair.  “One day Ian and I were in the lab. I shouldn’t say ‘one day.’ We’d been there pretty much nonstop for three days, running an experiment. And we got bored, so we started poking around the lab. And we found a really old lab manual, in my advisor's office.  It had the craziest experiments in there. One of them was ‘How to raise the dead.’”
    “And you tried it?” asked Lisa.
    “We thought it was a joke,” said Sarah.  “And we ran the experiment, but I accidentally dropped the flask out the window.”
    “How?” asked Jack.
    “Well, we were joking that we needed some test subjects, and there were a bunch of tourists outside, and you know, I was just holding the flask out of the window, and my hand was kind of wet...”
    “Four years ago?” asked Jack. 
    Sarah flinched.  “Yes.”
    “You’re saying, the reason that I’m-- whatever it is I am-- is because of a stupid college prank?”
    Sarah smiled weakly at him.
    He sprang to his feet and started pacing as Sarah shrank back into the chair.  Not something supernatural after all.  Just an experiment gone very strange. 
    He'd misunderstood everything, hadn't he?  This wasn't a punishment for the way he'd been, for what he'd done to Sam.  Say he'd kept his mouth shut, gone home, drunk himself to sleep, woken up the next day with the world open to him.  Say he'd called his friend with the startup who wanted a media relations guy, and said, hey, when can I start?  He could have had a heart attack jogging on the Battery in his forties, or liver failure in his sixties, or died in his bed surrounded by twenty-seven crying grandchildren, and the result would have been the same.  Zombie City, only fatter and with less hair.  And if it was just dumb luck, and not some kind of retribution for his sins, that changed everything, didn't it? 
    Maybe he hadn't been as bad as he thought.  Maybe he could even go home.
    But he was getting lost in his own thoughts again, the way he always did.  “Why did you and your friend shoot me with a tranquilizer dart?”
    “A couple of months ago, Prof. Leschke-- he’s my advisor-- came into our office. He was furious. He made us show him the lab manual. And then he took it from us. I haven’t seen it since.  He put us to work.  Synthesizing antivirals, to stop the infection.  We had to find a test subject, to inject him with them, to see if, you know, he stopped being a zombie..."  She trailed off, clutching her tea.
    Jack wondered what had happened to their guinea pig, and whether he had something to do with the bite on Sarah's arm.  “Can I assume he wasn’t a volunteer?”
    “I’m not a bad person, Mr. Kershaw!” she snapped.