Acute respiratory infection is one of the main causes of human disease worldwide. In the developing countries, acute respiratory infections are the main cause of death in children <5 years old. When lower tract manifestations are predominant in children, viruses are the most common agents, whereas in adults, bacteria are the leading causes of this syndrome. Upper respiratory tract infections are mainly viral in origin in the general population. Respiratory tract infections and reinfections occur throughout life but are more frequent in childhood. Several viruses belonging to different families and genera can cause infections, and it is impossible to recognize the etiologic agent on clinical grounds alone. Virologic diagnosis is easy to perform and permits specific antiviral treatment, potential withdrawal of antibiotics, and recognition of an epidemic situation with institution of preventive measures when indicated.