These tiny arachnids measure only about half a millimeter in length and are commensals living with humans. They are so small that they can circulate in air currents. Dust mites consume and thrive on the tiny flakes of skin we shed continually. They lay small groups of 3-5 eggs. A female can live from 1 to 3 months and lay nearly 100 eggs during her lifetime. Millions can live in a single mattress. A two year old pillow can contain large numbers of dead mites and their droppings. They also thrive in carpets and any upholstery. Although they may seem disgusting, dust mites are not harmful to most people, although about 10% of people develop allergies to the mites, especially to their cast skins and faeces.
Everything you don't want to know about dust mites
Wikipedia: Dust Mites