The disease of respiratory system is the most common type of disease in children as well as in adults. The probability of infection is determined by two most important factors: age and exposition to infection. Once the infection occurs, the cause of its severity is complex and it is also determined by genetic and environmental factors. The main part of respiratory diseases is caused by virus infection. There are significant differences between the immunologic reaction to the infection in children and in adults. Both the virus and the defence reaction of the host play an important role in determination of the resulting damage. Methisoprinol, substance with immunomodulating and antivirus effect, can be an effective tool in decreasing occurrence, length and severity of respiratory diseases, if well indicated.