A miasma is a thick, unpleasant atmosphere or vapor that people once believed caused diseases. For centuries, doctors thought illnesses like cholera and plague spread through bad air rising from rotting garbage, swamps, and sewage. They called this poisonous fog miasma.

The miasma theory turned out to be wrong: we now know germs cause most diseases, not bad smells. But the theory wasn't completely foolish. Places with terrible odors often were unhealthy, just not for the reasons people thought. A city with open sewers and piles of waste did spread disease, but through contaminated water and disease-carrying insects, not through the stench itself.

Today we still use miasma to describe any oppressive, unpleasant atmosphere. You might say a miasma of disappointment hung over the losing team's locker room, or that a miasma of suspicion spread through school after someone's lunch money went missing. The word captures that feeling of something heavy and toxic in the air, even when nothing's actually there.