Bronchial asthma and asthma-like form of COPD often undergo exacerbations with symptoms of infection. Currently, there is a general agreement, that most of these infections that exacerbate asthma especially in children are caused by viruses. Several "common cold" viruses are known to cause these exacerbations (RSV, parainfluenza virus, rhinoviruses). To date, there is no certainty, if viruses exacerbate asthma alone or in combination with allergen. It is also unknown, whether they can induce primary asthma symptoms. There is a growing support for the opinion, that in infectious exacerbations of asthma also bacteria can play an important role. These include both typical as well as atypical bacterial strains (Chlamydia pneumoniae). The effectiveness of antibiotics and bacterial immunotherapy with vaccines seems to support the role of bacteria in asthma exacerbations. Another very important problem is related to the role of infectious agents in prevention of atopy. Many facts have been gathered supporting the so called "hygienic theory".