Inflammation interferes with the activity of bacteria and viruses, and serves to clear them from the body. In DPB, bacteria such as Haemophilus influenzae and Pseudomonas aeruginosa can cause the proliferation of inflammatory cells into the bronchiolar tissues. However, when neither bacteria are present with DPB, the inflammation continues for an as yet unknown reason.[4][5] In either circumstance, inflammation in DPB can be so severe that nodules containing inflammatory cells form in the walls of the bronchioles.[4][13] The presence of inflammation and infection in the airways also results in the production of excess mucus, which must be coughed up as sputum.[4][6] The combination of inflammation, nodule development, infection, mucus, and frequent cough contributes to the breathing difficulties in DPB.[4][5]