The main factor is an excessive sensitivity of the airways in the lungs as a result of inflammation. Someone with asthma has sensitive airways all the time, but only has symptoms when they are exposed to a trigger. The lining of the airways swells and produces mucus, and the muscles around the airways tighten up. The narrowing of the space for the air to get into the lungs causes symptoms. This can lead to any combination of cough, wheeze and breathlessness.
Treatment involves avoiding the triggers if possible, and using medicines to treat any symptoms.
During an attack of asthma, there is often a degree of hyperventilation that arises directly from the inflamed airways. An attack of asthma is frightening. This can create a state of panic in a few people. The excessive overbreathing which results can dry the lining of the airways, making them tighten even more. In this case, learning to recognise and control the overbreathing can help the asthma too.