These airways can react badly when you have a cold or other viral infections or when you come in contact with an asthma trigger, which is anything that irritates the airway and cause symptoms of asthma to appear.

Common triggers include colds or flu, cigarette smoke, exercise and allergies to things like pollen, furry animals or house-dust mites.

When someone with asthma comes in contact with a trigger, the muscles around the walls of the airways tighten so that the airway becomes narrower. The lining of the airways becomes inflamed and starts to swell.

Source: The National Asthma Campaign.