Why are flies attracted to bad smells?

The smells that are a warning of putrefaction to us are attractive to flies for just the same reason. When organic matter (animal or vegetable) starts to rot, it becomes soft and moist, and provides ideal breeding conditions for fly larvae. Female flies are attracted to rotting material, where they lay their eggs in batches (around 50 to 100 in the case of the house fly). The eggs can hatch within 12 hours, and the larvae burrow into the soft material and feed for several days before turning into pupae, which then produce adult flies. It can all take as little as 10 days from egg to adult.