PlaugeTale is an AU where a virus spread via pollution in Waterfall. The whole Underground was broken down and sick, and only Sans, Undyne, Alphys and Toriel survived. (And all ghosts like Mettaton and Napstablook, and of course Flowey.) Sans is very quiet, and you battle him when he goes mental in the Genocide route.
When you enter the underground, you find nobody in the ruins except a torn up flower. You walk out of the Ruins and enter Snowdin, where you encounter Infected Sans. He wears a Lime Green hoodie and has a Dark Green eye. His font is Comic Sans, but with a Dark Green tint to it. He is usually quiet, and tells the occasional pun, but is usually in a grumpy mood. He can teleport, and his reflexes are extremely fast. Just like how Undertale Sans loves Ketchup, Infected Sans has acquired a taste for Relish. He runs Grillby's because of Grillby's death, and sells French fries dipped in relish which give you 10 HP back. He has weapons called Gaster-Blasters, but he modified them to shoot a poisonous beam which lowers your health 20% faster. He is always bloody.
Before the Plague happened, everything was just like regular old Undertale. Since the virus spread, he watched all of his friends and family die until few were left. That's probably why he's grumpy.
He is very close with everyone, ever since his brother died. If any of them died he would lose his mind.