The Infection Failed

She was grinning like the heroine of a horror movie while pretending to be scared.
"What are you so afraid of?" I asked.
"I'm watching a zombie movie," she said.
"And what's a zombie?"
Without missing a beat, she answered,
"They're the walking dead. If they catch someone, they suck their blood, bite them, and the victim turns into one of them."
I couldn't help laughing.
"Funny... By that definition, you'd make a better zombie leader than the zombies themselves. You've been slowly sucking the life out of me for years, and every chance you get, you bite my head off. Yet here I am—not a zombie."
I paused, then added,
"Either my immune system is stronger than the virus... or even zombies have standards, and somehow I didn't make the cut."
She glared at me.
I, on the other hand, was trying to figure out one thing:
How does a zombie get angry when it realizes its victim isn't affected at all?