Asthma is characterised by intermittent bouts of wheezing, coughing and breathlessness, which are caused by the temporary narrowing of air passages leading to the lungs, and also to partial blockage of these passages by passages mucus or inflammatory swelling.
Infection plays a modest part, and fortunately stress is a very minor one.
The child comes in contact with something to which he is allergic and his air passages start closing down almost as if making a desperate attempt to keep out the foreign material. If your child has asthma, you can begin by trying to identify the allergen. You may not succeed, but it's worth a try. And these are causes, which you can do something about by keeping the child away from them. In very sever cases where there is no other solution, it may even be necessary to move to a house with less dust.