A viral infection is nearly often the cause of bronchiolitis. In most cases, the culprit is a respiratory syncytial virus (RSV). RSV is a very common virus, and almost all children are infected by the age of 2.
Both are lung conditions with similar symptoms, so it can be difficult to tell the difference. However, each of them affects different parts of the lungs: Bronchitis affects the bronchial tubes that carry air to the lungs. Pneumonia affects the air sacs, called alveoli, where oxygen passes into the blood.
In many clinical studies, all wheezing illnesses other than asthma in children younger than 3 years have been diagnosed as bronchiolitis. Asthma is a chronic inflammation of the airways and, clinically, an acute asthma attack simulates bronchiolitis.