Asthma is a disease of the lungs, where the muscles around your air passages in your lungs clamp up, swell up and get inflamed. Normally these passages (called bronchioles) whizz air (with oxygen) into the air sacs in your lungs when you breath in, and take the used air (full of carbon dioxide) out when you breath out. Clamping up makes the bronchioles smaller so air can't get in or out so easily. Getting the air OUT is the most difficult bit when you have asthma. About 1 in 10 of all people will get asthma at some time or another - usually when they are young. Often it is very mild but an asthma attack can be scary if it's severe - you can't breathe, you feel you're suffocating and you wheeze like a broken pipe organ.
About 1 in 10 of all people will get asthma at some time or another.