"You may think that you're dirt-free after a bath or a shower, but the human body is never truly clean. Your skin supports a host of icky things----such as yeast and bacteria---and your bath towel absorbs them when you're drying yourself. Furthermore, a lot of your surface skin is made up of dead skin cells, which cling to you unless they're scraped off by an abrasive object such as a towel or a brush: This gunk collects in your towel. The air around you in your home also contains dead skin, dirt, microscopic bits of dead bugs, and other debris. So even when your towel is innocently hanging from its hook, it's getting dirty.
By the way, if your towels continue to have a musty smell after you launder them, it might be due to mildew in your washing machine. Your washer may retain water, which stagnates and results in mildew. Because bath towels are so absorbent, they're more obviously affected than other items.
So give your washing machine the once-over and toss your towels in there on a regular basis. Staying clean is a never-ending battle."
From the book;
"Why Do Men Leave The Seat Up?"