Respiratory syncytial virus (RSV) usually causes cold-like symptoms but can also cause more severe diseases, such as pneumonia and bronchiolitis. Bronchiolitis, an inflammation of the small airways in the lung with increased mucus production and bronchospasms (contraction and relaxation of the small airways), most often occurs in children younger than 2 years. In children, RSV infections may start out with fever, mild cough, and congestion. This can lead to worsening cough and labored and rapid breathing that interfere with the ability to eat and drink. In the United States, RSV infections usually occur between November and May but may occur earlier in the year in some areas. The infection spreads by contact with an infected person's saliva and mucus containing the virus.