Bronchiolitis is a common disease of the lower respiratory tract. A specific virus called respiratory syncytial virus (RSV) may cause bronchiolitis. It is most common in the winter and mostly in children under 2 years of age. Bronchiolitis begins like a cold (runny nose and sneezing) and within days causes coughing, wheezing, and breathing trouble. This can also make it hard for the child to eat and often makes them uncomfortable and cranky. Mild cases, especially those with thick nasal mucus, may get some relief with a cool-mist vaporizer. A child who has a lot of trouble breathing may need to go to the hospital for oxygen.