Periods of acute deterioration known as exacerbations significantly lower the quality of life and health status of patients with chronic obstructive pulmonary disease (COPD), thus making treatment of these patients more expensive. Understanding of the pathogenesis of COPD exacerbations and their influence on the clinical course and prognosis of this disease has greatly improved during the last decades. Some aspects of the interrelation between the infectious pathogen and the host organism have been studied. According to modern data, respiratory infections present one of the main causes of exacerbations of the disease. The role of respiratory viruses, especially rhinoviruses and respiratory syncytial virus, which often precede secondary bacterial infections, have been found to be important in the etiology of COPD exacerbation. The main bacterial etio-pathogens of COPD exacerbations--the non-capsulated bacteria H. influenzae, S. pneumoniae, and M. catarrhalis, which in 25 to 50% of cases colonize the lower respiratory tract (LRT)--have been isolated. Colonization of the LRT of infectious agents (viruses and bacteria) results in chronization of the inflammatory process and progress of the disease. Besides, colonization correlates with the severity of the disease. A "vicious circle" theory has been put forward to explain the pathogenesis of COPD exacerbations. Etiological diagnostics of a COPD exacerbation will allow adequate choice of effective etiotropic therapy making it possible to eradicate the pathogen or lower microbial load in the respiratory tract, which will decrease the risk of recurrence and improve the prognosis of the disease.