How its contracted

Asthma can arise at any age, but why some people have the disease and others don't is not known. People with asthma have airways that are more sensitive than normal.

> Doctors know, however, that asthma can sometimes run in families.

> Asthma attacks can be set off by many different things, these are called triggers. Examples include cold air, vigorous exercise and stress.

> These triggers may also include 'allergens'. These are present in the environment and contain chemicals that trigger allergic reactions.

> Allergens include, for example, pollen, animal danders, house dust, pollution, some foods, perfumes and cigarette smoke.

> Allergens cause the lining of the airways to become swollen and inflamed. It produces extra mucus and the muscles of the airways tighten. There is then less room for the air to pass in and out.

> Attacks may be more frequent or severe in people who have a chest infection.