Bronchiolitis is a common illness of therespiratory tract caused by an infection that affects the tiny airways, called the bronchioles, that lead to the lungs. As these airways become inflamed, they swell and fill with mucus, which can make breathing difficult.

Occcurs to

  • most often affects infants and young children because their small airways can become blocked more easily than those of older kids or adults
  • typically occurs during the first 2 years of life, with peak occurrence at about 3 to 6 months of age
  • is more common in males, children who have not been breastfed, and those who live in crowded conditions

Childcare attendance and exposure to cigarette smoke also can increase the likelihood that an infant will develop bronchiolitis.

Although it's often a mild illness, some infants are at risk for a more severe disease that requires hospitalization. Conditions that increase the risk of severe bronchiolitis include prematurity, prior chronic heart or lung disease, and a weakened immune system due to illness or medications.

Kids who have had bronchiolitis may be more likely to developasthma later in life, but it's unclear whether the illness causes or triggers asthma, or whether children who eventually develop asthma were simply more prone to developing bronchiolitis as infants. Studies are being done to clarify the relationship between bronchiolitis and the later development of asthma.

Bronchiolitis is usually caused by a viral infection, most commonlyrespiratory syncytial virus (RSV). RSV infections are responsible for more than half of all cases of bronchiolitis and are most widespread in the winter and early spring. Other viruses associated with bronchiolitis include rhinovirus, influenza (flu), and human metapneumovirus.