The answer to 'humidifier smells bad' is that a bad smell coming from your humidifier is usually caused by bacterial growth or mold. This can happen when the humidifier is not cleaned and maintained regularly. Symptoms may include a musty, stale, or unpleasant odor coming from the unit.
The solution is to clean and disinfect the humidifier thoroughly, including the water tank, filters, and any other internal components. This is generally a DIY task, but if the smell persists or you're unsure how to properly clean the unit, it's best to call in a professional HVAC technician to inspect and service the humidifier.