For the first few days, the signs and symptoms of bronchiolitis are similar to those of a cold:

• Runny nose
• Stuffy nose
• Cough
• Slight fever (not always present)

After this, there may be a week or more of difficulty breathing or a whistling noise when the child breathes out (wheezing).

Many infants will also have an ear infection (otitis media).

Bronchiolitis occurs when a virus infects the bronchioles, which are the smallest airways in your lungs. The infection makes the bronchioles swell and become inflamed. Mucus collects in these airways, which makes it difficult for air to flow freely in and out of the lungs.

Most cases of bronchiolitis are caused by the respiratory syncytial virus (RSV). RSV is a common virus that infects just about every child by the age of 2. Outbreaks of the RSV infection occur every winter. Bronchiolitis can also be caused by other viruses, including those that cause the flu or the common cold. Infants can be reinfected with RSV because at least two strains exist.

The viruses that cause bronchiolitis are easily spread. You can contract them through droplets in the air when someone who is sick coughs, sneezes or talks. You can also get them by touching shared objects — such as utensils, towels or toys — and then touching your eyes, nose or mouth.

Infants younger than 3 months of age are at greatest risk of getting bronchiolitis because their lungs and immune systems aren't yet fully developed.

Other factors that are associated with an increased risk of bronchiolitis in infants, or more severe illness due to bronchiolitis, include:

• Premature birth
• An underlying heart or lung condition
• A depressed immune system
• Exposure to tobacco smoke
• Never having been breast-fed — breast-fed babies receive immune benefits from the mother
• Contact with multiple children, such as in a child care setting
• Living in a crowded environment
• Having siblings who attend school or child care and bring home the infection

Complications of severe bronchiolitis may include:

• Blue lips or skin (cyanosis). Cyanosis is caused by lack of oxygen.
• Pauses in breathing (apnea). Apnea is most likely to occur in premature infants and in infants within the first two months of life.
• Dehydration.
• Low oxygen levels and respiratory failure.

If these occur, your child may need hospitalization. Severe respiratory failure may require that a tube be inserted into the trachea to help the child's breathing until the infection has run its course.

If your baby was born prematurely, has a heart or lung condition, or has a compromised immune system, watch closely for beginning signs of bronchiolitis. The infection can quickly become severe. In such cases, your child will usually need hospitalization.