These little bugs are so small that between 100 and 500 individual mites live in a single gram of dust. Despite their size, dust mites cause some big problems. Or more specifically, their feces do. Related to spiders and ticks, dust mites live in fabrics like bedding and carpeting that collect dust. While we used to believe that people were allergic to dust, researchers now know it's not the dust itself, but the mites that live in it that cause the problem.