It started with a smell. Not strong at first — just an odd, sour note that drifted through the hallway every now and then.

Tom Fisher, a 42-year-old homeowner in the suburbs, noticed it one evening while making dinner. He figured it was something in the trash, maybe an onion gone bad or a bit of spoiled food left behind the stove. He cleaned the kitchen top to bottom, took out the garbage, and opened the windows. By morning, the smell was back.

At first, Tom tried to ignore it. The odor seemed faint, almost ghostlike — appearing at random and disappearing before he could trace its source. But within a few days, it had grown into something sharp and foul, like rotting meat mixed with mildew.