House dust mites are microscope bugs that primarily live on dead skin cells regularly shed from humans and their animal pets. Dust mites are harmless to most people. They don't carry diseases, but they can cause allergic reactions in asthmatics and others who are allergic to their feces.
Skin cells and scales, commonly called dander, are often concentrated in lounging areas, mattresses, frequently used furniture and associated carpeted areas, often harbor large numbers of these microscopic mites. Since the average human sloughs off 1/3 ounce (10 grams) of dead skin a week. That gives dust mites a lot to eat. Cats and dogs create far more dander for dust mites to eat.
A typical mattress can contain tens of thousands of dust mites. Sick yet? Nearly 100,000 mites can live in one square yard of carpet. Ready to convince your spouse to start bathing regularly? Did you know a single dust mite produces about 20 waste droppings each day, each containing a protein to which many people are allergic. Yuck! The proteins in that combination of feces and shed skin are what cause allergic reactions in humans. Depending on the person and exposure, reactions can range from itchy eyes to asthma attacks. And finally, unlike other types of mites, house dust mites are not parasites, since they only eat dead tissue. Gross, but true.