Washing machine odors are caused by mold, mildew or bacteria that occurs due to buildup of detergent, dirt and water in the machine. This buildup may be accelerated by the design of the washer and certain laundry habits such as overuse of laundry products.
Front-loading washing machines are more likely to have odors than top-loading ones. The door seals of front-loaders are usually airtight to prevent leakages, and closing the door after removing the laundry traps moisture inside the washing drum, creating an optimum environment for mold, mildew and bacteria. Foul washer smells can be prevented by running a cleaning cycle with just hot water and bleach once a month and leaving the door open when the machine is not in use.