A bad smell, or odor, is an unpleasant scent that can arise from various sources. Common causes include decaying organic matter, spoiled food, or certain chemicals. Odors can be detected by the human nose and can trigger strong emotional responses or memories. Bad smells can also indicate potential health hazards. For example, a foul odor from sewage may suggest a plumbing issue, while a rotten smell from food can signal spoilage. Identifying and addressing the source of a bad smell is important for maintaining a healthy and pleasant environment.