Viral infections are the most common cause of acute respiratory diseases. They are caused by adenoviruses, rhinoviruses, influenza and parainfluenza viruses, metapneumoviruses, syncytial-respiratory viruses and coronaviruses. Respiratory symptoms also may be due to hemorrhagic fever viruses, especially hantaviruses, that not occur in Poland. The clinical course of viral infections varies from asymptomatic to rapidly-progressing, high- mortality disease. The diagnosis is based on serology and genetic methods. It is also possible to isolate the virus in the tissue culture. Virus-specyfic treatment is not available, except influenza virus.