Visualization of a bronchiole that is severely swollen and filled with mucus. These symptoms are caused by asthma, which develops when there is a blockage of air flow within the lungs. This blockage is most often caused by an excess of mucus (inflammatory cells) that show up to heal the bronchioles when they are damaged from allergy, infection, or injury. When the bronchiole pathways fill with mucus, irritation and swelling occur thus there is little room left for air. Breathing then becomes difficult and may be accompanied by wheezing or coughing.