Bath mats, left damp, can develop mildew which can be incredibly dangerous to your health. “Although you may only step on bath mats when you are fresh out of the bath or shower, bath mats are a breeding ground for bacteria and germs,” warns French, “as they retain water, which is normally warm, bath mats can develop mildew and, in some circumstances, can cause bacterial and fungal growth.” Washing them once a week will help keep them clean and hygienic.
Article source: https://www.huffingtonpost.co.uk/entry/how-often-to-clean-room-by-room_uk_61efcb3de4b00e5113d6fc60