 Most cases of bronchiolitis are caused by the respiratory syncytial virus (RSV). This virus thrives in the human nose and throat, and causes symptoms to appear a few days after the initial infection. Other examples of viruses that may cause bronchiolitis are the parainfluenza virus, and the influenza (flu) virus.
Viruses are spread through either direct contact or through tiny airborne droplets of liquid, passed on through coughing and sneezing. Children who have had bronchiolitis can still be infectious for up to three weeks after having the condition, even after their infection has cleared.
Once the virus enters the respiratory system through the windpipe (trachea), it makes its way down to the smallest airways in the lungs, which are called the bronchioles. The infection causes the bronchioles to become swollen and blocked with mucus, which hinders breathing. Babies and young children have tiny, often under developed airways, which is what makes them prone to bronchiolitis.
Children who are born with lung or heart diseases, those who are born prematurely, and those with weakened immune systems are more at risk of developing bronchiolitis. There is also some evidence that babies who are never breast fed, and those that are exposed to cigarette smoke may be more susceptible to the condition. | 
