<qa>  What is the leading cause of respiratory tract infections in infants and young children?Globally, Respiratory Syncytial Virus RSV is recognized as the leading cause of respiratory tract infections in infants and young children and a major cause of hospitalization due to severe respiratory infection. Despite four decades of effort, there are still no effective methods to control RSV infection. Treatment of RSV has been limited to supportive measures. There is an urgent need for safe and effective drugs to treat and prevent RSV disease. 