Unsurprisingly, bacteria and dust mites can cause or worsen allergies, targeting a weakened immune system."The three main things you want to avoid on your bedding are dust mites, mold, and mildew, since they activate allergies or worsen existing ones," says Dr. Carl Cricco, co-founder of the snuggL, a new L-shaped pillow that helps prevent back pain. Invest in bedding made from hypoallergenic materials, which wick away warm moisture so mold and mites can't breed. Washing your linens will kill any bacteria that's already on them, but make sure to replace synthetic pillows every two to three years and a down or feather pillow every five to six years so bacteria or bugs don't have a chance to build up. You can also wash most down or synthetic pillows as well — just make sure to check the care label before doing so.