He should have known better than to tell his wife she couldn't do something.
Ever since they'd met, he'd always realize - just a little too late - that he had figuratively dangled the carrot in front of her nose. She had always been a nosy person, a curious person, ever since his cousin introduced him to her, and sometimes that got her into trouble. Of course, he'd always forgiven her for her intrusiveness, figuring it had to do with her upbringing, the fact she seemed to spring forward as a fully formed person from the ether. Her family, as far as he knew, was nonexistent, which he'd considered a boon - no in-laws to contend with.
But his brother had always warned him to be attentive to her needs. And he'd agreed wholeheartedly to do just that. He did think his brother could be a little too presumptuous, considering he also warned him to keep a keen eye on her, too.
She was the love of his life. What could she do to hurt him?
And then, his cousin gave him a box to keep. A simple cardboard box, packaged with tape, sealed. He had said he needed him to just keep it for him for a little while. His cousin never said what was in it, and frankly he didn't want to know. Knowing him, it was some aphrodisiac, a cancer of lust in the form of nudie mags and masturbatory aids of all kinds. There just some things people were better off not thinking about.
But she asked him about the box, and he said she couldn't open it, and just as he'd said those words, he knew he'd made a mistake. It was too late by then. He couldn't take the words back.
And so, when he arrived home, he found her with the open box, empty but for a single thing.
"I think I let something out," she sobbed. "I-I didn't know. I just..."
And he knew what she would say.
"...was curious."
So he knelt beside her and found, in the box, a bunch of vials that had opened, labelled with frighteningly scientific names. All that was left was a single vial reading potential vaccine.