Respiratory syncytial virus (RSV) is the most common cause of bronchiolitis in children younger than 1. RSV usually only causes mild symptoms in adults, but it can cause severe illness in infants. It is estimated that by the end of their first year, more than half of all infants have been exposed to RSV.

In older children, research has shown that rhinovirus is the most common cause of bronchiolitis. Other viruses that have been implicated include adenovirus, influenza, and parainfluenza. In some cases, bronchiolitis may be caused by multiple infections, such as a combination of RSV and influenza.

The infant may have severe respiratory distress and become cyanotic. Young infants may become exhausted from the work of breathing and require endotracheal intubation. Secondary infection, such as pneumonia, may also occur. In severe cases of bronchiolitis, and when treatment has been delayed, the infection may be fatal.

Later in life, the patient may develop airway disease, such as asthma.

Bronchiolitis often starts as a mild upper respiratory infection with symptoms similar to those of a common cold, such as fever, rhinorrhea, and cough. Over the course of 2 or 3 days, however, increased respiratory distress can develop, along with wheezing and a tighter cough. Lung sounds may reveal the presence of crackles and a high-pitched expiratory wheeze. The infant may be tachypneic, hypoxic, and irritable. Nasal flaring, grunting respirations, and use of accessory muscles signal worsening respiratory distress. (See Observing retractions.)

The chest may also appear hyperinflated. Affected infants may also be lethargic and dehydrated due to poor feeding.