A miasm is a bad smell or a cloud of bad air that can make people sick.
Miasms are usually thought of as very bad and unhealthy.
Examples:
The miasm from the swamp made everyone sick.
I can smell the miasm from the garbage.
The miasm hung in the air like a dark cloud.
The miasm in the bathroom was so bad, I had to leave!
The miasm was so terrible, it made the birds fall from the sky.
The miasm was like a monster, creeping into our noses and making us feel sick.