Cat Lady
I rented one side of a duplex and my landlady lived in the other half. She was a cat hoarder who had turned three of her bedrooms into giant litter boxes. She even covered the beautiful hardwood with garbage bags and dumped several hundred pounds of cat litter onto the floor. She would go weeks without cleaning these “litter boxes.”
She would also put cans of wet cat food and tuna in the backyard for the feral cats. The food would go bad and the smell of rancid cat food and festering cat pee was too much to handle—but the final straw was yet to come. That happened when she took in a cat that had the extremely contagious feline parvovirus. It spread like wildfire and took the lives of 23 of her 78 (!) cats.
Unbeknownst to me, several sick cats had puked, peed, pooped, and died on our shared porch. Because we were unaware that there was a Parvo outbreak next door, we tracked the virus into our house and it killed our favorite cat. My then-fiancé was incensed at our landlady. We ended up moving out a few weeks later. RIP Max.
Story credit: Reddit / MedicGirl