The answer to 'grout smells bad' is that a bad odor in grout is usually caused by mold, mildew, or bacteria growth. This can happen when the grout becomes damp and doesn't dry out completely, providing a breeding ground for these microorganisms. Symptoms include a musty, unpleasant smell and potentially visible discoloration or staining of the grout.
To fix this issue, you'll need to thoroughly clean and disinfect the grout. Start by scrubbing the grout with a stiff-bristled brush and a mixture of warm water and vinegar or a specialized grout cleaner. You may need to let the solution sit for several minutes before scrubbing to allow it to penetrate. Once the grout is clean, apply a grout sealer to help prevent future moisture buildup and microbial growth.