Parainfluenza viruses are a group of four distinct serotypes of single-stranded RNA viruses belonging to the paramyxovirus family. They are the second most common cause of flulike respiratory tract infection in younger children.The viruses are transmitted by droplet infection. Mainly affected are small children till the age of 3 years in temperate climate zones. A seasonal accumulation of infections in cool seasons can be recognized. Generalized contamination levels in the population are already high in childhood, 90% in 10-year-old children.Parainfluenza viruses primary attach the pharyngeal mucosa, but can spread on the whole tracheobronchial tract. Heavy inflammatory reactions with the distribution of proinflammatoric zytocines occur, may accompanied by an epiglottis. The excessive mucous secretion may leads to the obstructions of the respiratory tract - especially in younger children.Parainfluenza viruses type 1 and 3 cause a flulike infection in small children accompanied by fever, cough, rhinitis, bronchitis and pseudo croup. Pneumonias can occur. Bacterial superinfections are frequent, as are the usually harmless reinfections.Infections by Parainfluenza viruses type 2 lead to similar clinical symptoms like with the type 1 and 3, the course of infection is milder.