With every wash, your washing machine collects detergent residue, dirt, and your dog or cat's hair. In a closed, damp environment like that of a modern washing machine—with hermetically sealed doors and less water than those of twenty years ago—these residues, along with natural animal hair oils, humidity, and mold, create the perfect conditions for one particular bacterium.

It's called biofilm. It has an invisible bacterial structure composed of a protein shield that prevents common solutions from destroying its structure. That's why if you tried to eliminate the odor, it returned after a few days.