We've all been too lazy to wash our sheets. However, we really should put them in the washer, because they can get very dirty, very fast. According to a study, if a pillowcase isn't washed regularly enough, it could be covered in several thousand times more bacteria than a toilet bowl.In fact, every night we shed 15 million dead skin cells on our pillowcase. These cells will not only accumulate on the pillowcase, but they will also feed the hundreds of thousands of mites present on it. The less the pillowcase is washed, the more dead skin it will have to feed the mites, and the more these mites will reproduce.In addition, the multiplication of mite excrements will cause a rapid increase in bacteria and fungi on your pillowcase.Dangerous for your healthIf you have an allergy to dust mites, not washing your pillowcase can be dangerous for you. They produce proteins that irritate your eyes, make your nose run, and cause other symptoms you might mistake for a cold.According to one study, an unwashed pillowcase has 16 different species of fungi that are potentially harmful to the lungs and other organs.How often should you wash your sheets?That is the question scientists sought to answer, and the results are out! In order to limit the number of bacteria on them, sheets should be washed once a week, in 60 ° C water.