The answer to 'grout smells bad' is that the most common causes are mold, mildew, or bacteria growth in the grout. This can happen due to moisture, poor ventilation, or lack of regular cleaning. Symptoms include a musty, unpleasant odor coming from the grout. The solution is to clean and disinfect the grout, and address the underlying moisture issue.
For minor grout odors, you can often address this with a DIY grout cleaning. Use a grout brush, grout cleaner, and disinfectant. However, if the odor is severe or the grout is badly damaged, it's best to call in a professional cleaning service to properly clean and reseal the grout.