Smell, like the other human senses, is subjective. However, there are a few smells that many people seem to find particularly unappealing. These smells include human feces, rotting meat and animal feces.
A scent that smells bad to one person may not smell bad to another. But some smells are widely acknowledged as horrible or unpleasant. The smells of skunk, rotting flesh or vomit, human or otherwise, are generally acknowledged to be bad smells. There is a good reason these things smell bad to people: our sense of smell tells us if something is good or bad to eat.
Scientists have found that peoples' sense of smell differs depending on their genetics. This means that two people probably perceive slightly different smells from a piece of cheese, for example. However, this does not apply to truly bad smells, like rotting meat. Scientists have found that evolution has taught our brains to avoid the smell of rotting meat, as eating it is likely to make us sick. The same holds true of human excrement or vomit. Many poisonous plants and substances also have unpleasant smells. Often, scientists say, a bad smell is a warning that something may be dangerous to ingest.