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. In either circumstance, inflammation in DPB can be so severe that nodules containing inflammatory cells form in the walls of the bronchioles. The presence of inflammation and infection in the airways also results in the production of excess mucus, which must be coughed up as sputum. The combination of inflammation, nodule development, infection, mucus, and frequent cough contributes to the breathing difficulties in DPB.The fact that inflammation in DPB persists with or without the presence of P. aeruginosa and H. influenzae provides a means to determine several mechanisms of DPB pathogenesis. Leukotrienes are eicosanoids, signaling molecules made from essential fatty acids, which play a role in many lung diseases by causing the proliferation of inflammatory cells and excess mucus production in the airways. In DPB and other lung diseases, the predominant mediator of neutrophil-related inflammation is leukotriene B4, which specializes in neutrophil proliferation via chemotaxis (the movement of some types of cells toward or away from certain molecules).Inflammation in DPB is also caused by the chemokine MIP-1alpha and its involvement with CD8+ T cells. Beta defensins, a family of antimicrobial peptides found in the respiratory tract, are responsible for further inflammation in DPB when a pathogen such as P. aeruginosa is present.
Physicians who examine persons with ILI can use a combination of epidemiologic and clinical data (information about recent other patients and the individual patient) and, if necessary, laboratory and radiographic tests to determine the cause of the ILI. The use of multiplexed point-of-care testing such as CRP (C-reactive protein) along with an examination by a doctor may help to identify a bacterial and avoid an unnecessary antibiotic prescription.During the 2009 flu pandemic, many thousands of cases of ILI were reported in the media as suspected swine flu. Most were false alarms. A differential diagnosis of probable swine flu requires not only symptoms but also a high likelihood of swine flu due to the persons recent history. During the 2009 flu pandemic in the United States, the CDC advised physicians to "consider swine influenza infection in the differential diagnosis of patients with acute febrile respiratory illness who have either been in contact with persons with confirmed swine flu, or who were in one of the five U.S. states that have reported swine flu cases or in Mexico during the 7 days preceding their illness onset." A diagnosis of confirmed swine flu required laboratory testing of a respiratory sample (a simple nose and throat swab). In rare cases
If a person with ILI also has either a history of exposure or an occupational or environmental risk of exposure to Bacillus anthracis (anthrax), then a differential diagnosis requires distinguishing between ILI and anthrax. Other rare causes of ILI include leukemia and metal fume fever.