Asthma, in many people, appears to be an allergic reaction to substances breathed in through the air.  These are allergans, which is something that provokes an allergic reaction and includes things such as pollen, dust mites, animal dander.  An asthma attack can occur when a person breaths in these allergens, this can send the immune system in overdrive as if fighting off a harmful parasite.  The airways can become inflames and constricted, leading to coughing, wheezing, and breathing difficulties.

(more details/links will be added shortly)