Exacerbations of asthma and COPD are a major cause of morbidity, mortality and economic burden to the patient and society. Exacerbations are usually associated with worsening of airway inflammation which, as identified by quantitative sputum cell counts, can be neutrophilic, eosinophilic, combined eosinophilic and neutrophilic or neither, a result of different causes. The commonest cause of exacerbations is infection. Neutrophilic exacerbations are common and are usually associated with bacterial and non-bacterial infections but eosinophilic exacerbations might also predict viral infections. However the investigators are still unsure whether aspects of neutrophilic inflammation can differentiate viral from bacterial infections. Also the exact relationship between etiology, mechanisms of susceptibility to infection, inflammation and airway responsiveness during an exacerbation is poorly understood. As a result, the physician assumes that clinical improvement from an exacerbation corresponds to resolution of infection and underlying inflammation of airways at the same time. However, inflammation of the airways may persist beyond the resolution of infection, which may contribute to the morbidity and mortality of the disease.