Why is carbon monoxide called the silent killer?

It is very dangerous to sleep in a room where coal or wood is burning and its doors and windows are closed. Owning to the limited supply of air in such a room, carbon monoxide is produced. Since the gas is colourless and has a barely detectable smell, people sleeping in the room do not feel its presence and run the risk of carbon monoxide poisoning. That is why carbon monoxide is also called a silent killer.