Dust mites are a common problem with dust. My brother is allergic to them and they can restrict his breathing and generally make him feel very flu-ish and lethargic. It takes extensive cleaning to rid the house of them. "A scoop of dust weighing as much as a paper clip contains as many as 19,000 dust mites." (WebMD) "Exposure to dust mites, animal dander, cat saliva and mold causes about 200,000 emergency-room visits a year for people with asthma." (Aerias) I remember once we had to totally take my brothers room apart, and he had to sleep in my room for a month or two while my mother steam cleaned every inch of his room and everything that had been in it.

Dust mites aren't the only thing caused by dust. People with breathing trouble or heart problems can experience complications in breathing due to compacted dust. Dust can also trigger eye problems for people with or without allergies. It can irritated anyone's lungs, depending on the amount. It can also cause skin problems such as itching and rashes.

Dust is always contaminated to micro-organism that causes disease of respiratory tract.Dust may causes allergy in skin(itching,skin rash,urticaria),allergy in nose(allergic rhinitis),allergic asthma.
Mycobacterium tuberculosis,organism of tuberculosis, may present in dust,if one inhale that and immunity present low,that person may develop tuberculosis.So physician always give advise to avoid dust,using carpet in room who suffered from asthma and allergy as preventive measure.