An unwashed pillowcase can host up to 3 million bacteria after just one week, according to a new report by Amerisleep. That is about 17,000 times more bacteria than found on the average toilet seat. The bacteria on pillowcases can include allergens, fungi and dead skin cells that attract dust mites and other microscopic critters. "Dead skin cells and sweat provide food for dust mites, attracting them to your bed and helping them multiply," said Dr. Hadley King. Experts warn that pillowcases, specifically, should be changed routinely and more frequently than once a week. A recent survey from the UK discovered that nearly half of men wait up to four months to change their sheets.