Bronchiolitis is a medical condition where the smaller parts of the lung (called bronchioles) are irritated. This makes it very hard to move air in and out of the lungs. Many things can cause bronchiolitis. The most common is the respiratory syncytial virus. The disease Bronchiolitis is not the same as Bronchitis. The medicine used to treat bronchitis may not be the same as those used to treat bronchiolitis. The disease is more common in children.