Bronchiolitis is a viral lung infection that inflames and congests the smallest airways (bronchioles) in the lungs. It almost exclusively affects children under two years old, and respiratory syncytial virus (RSV) causes roughly 77% of cases. Most children recover at home within one to two weeks, but the illness can cause significant breathing difficulty, especially in very young infants whose airways are naturally tiny.
What Happens Inside the Lungs
The infection typically starts in the nose and throat, where the virus replicates for a day or two before spreading down into the lower airways. Within one to three days of that initial cold-like phase, the virus reaches the bronchioles, the smallest tube-shaped passages deep in the lungs. There, it damages the cells lining the airway walls and triggers an inflammatory response.
That inflammation causes three problems at once: the airway walls swell, the lungs produce excess mucus, and dead cells slough off into the already-narrowed passages. In an adult, this might cause a mild cough. In an infant, though, those bronchioles are incredibly small to begin with. Even modest swelling can partially or fully block airflow, trapping air in parts of the lung and reducing oxygen levels. This is why bronchiolitis produces wheezing, rapid breathing, and visible effort to breathe.
Which Viruses Cause It
RSV dominates. In a large study of nearly 3,000 hospitalized infants across multiple U.S. medical centers, RSV was found in 76.5% of cases. Rhinovirus (the common cold virus) came second at 23.8%, followed by coronavirus (6.9%), adenovirus (6.4%), and human metapneumovirus (6.0%). These five viruses together accounted for more than 90% of severe cases. Some infants test positive for two viruses at once, which is why those percentages add up to more than 100.
How Symptoms Progress
Bronchiolitis almost always begins looking like a normal cold: runny nose, mild cough, possibly a low fever. For the first two or three days, it can be hard to tell anything more serious is happening. Then, as the virus moves into the lower airways, breathing symptoms develop. You may notice your child breathing faster than usual, wheezing (a high-pitched sound when breathing out), or coughing more deeply.
Symptoms typically peak around day three to five of the illness, then gradually improve. The cough often lingers for two weeks or longer. Feeding difficulties are common during the peak days because babies need to pause breathing to swallow, and when breathing is already labored, eating becomes exhausting.
Signs That Need Urgent Attention
Most bronchiolitis is manageable at home, but certain signs indicate the illness is becoming more serious. Watch for retractions, which is when the skin pulls inward between the ribs, below the ribcage, or at the base of the throat with each breath. The more locations you can see retractions, the harder your child is working to breathe. Nasal flaring (nostrils widening with each breath) and grunting sounds are also warning signs.
Poor feeding is one of the strongest predictors of a child needing hospital care. If your baby is taking less than half of their normal feeds, hasn’t had a wet diaper in six to eight hours, or seems unusually sleepy and hard to rouse, those are reasons to seek care promptly. Bluish color around the lips or fingernails means oxygen levels have dropped and requires immediate attention.
How It’s Diagnosed
Bronchiolitis is diagnosed based on what the doctor sees and hears during a physical exam, combined with the child’s age and the time of year. The American Academy of Pediatrics specifically recommends against routine chest X-rays or lab tests for typical cases. Imaging is generally reserved for children sick enough to need intensive care or when doctors suspect a complication like a collapsed portion of the lung. Routine virus testing isn’t recommended either, since identifying the specific virus rarely changes treatment.
Treatment Is Mainly Supportive
There is no medication that reliably shortens bronchiolitis or reduces its severity. Bronchodilators (the type of inhaler used for asthma) and steroids are widely prescribed in practice, but the evidence doesn’t support routine use. Meta-analyses of inhaled bronchodilators have failed to show consistent benefits, and large studies of steroid treatment found no meaningful difference in hospital admission rates or symptom scores compared to placebo.
What actually helps is keeping the airways clear and the child hydrated. Nasal suctioning removes mucus from the nose so the baby can breathe and feed more easily. In hospitals, suctioning happens every one to four hours around the clock. At home, you can use a bulb syringe or nasal aspirator before feedings and before sleep. Saline drops in the nose before suctioning help loosen thick mucus. Keeping the child’s head slightly elevated and offering smaller, more frequent feeds also helps. Supplemental oxygen is given in the hospital when blood oxygen levels drop below a safe threshold.
Who Is at Highest Risk
Premature infants face the greatest risk of severe bronchiolitis because their lungs and immune systems are less developed. Babies born at 33 to 34 weeks of gestation have notably higher hospitalization rates than those born closer to full term. Beyond prematurity, children with chronic lung disease, congenital heart defects, immune deficiencies, and neuromuscular disorders are also more likely to need hospital care. Very young age matters too: infants under three months old are more vulnerable simply because their airways are at their smallest.
RSV Prevention Options
Because RSV causes the vast majority of bronchiolitis, prevention efforts focus on this virus. Two main strategies now exist. The first is maternal vaccination: a pregnant person can receive the RSV vaccine during pregnancy so that protective antibodies pass to the baby before birth. The second is a monoclonal antibody injection given directly to the infant. These antibodies circulate in the baby’s blood and neutralize RSV before it can establish a serious infection.
The CDC recommends an RSV antibody for infants younger than eight months who are born during or entering their first RSV season (typically fall through spring) if the mother did not receive the RSV vaccine during pregnancy, if her vaccination status is unknown, or if the baby was born within 14 days of vaccination. The dose depends on the baby’s weight: 50 mg for infants under about 11 pounds, 100 mg for those 11 pounds and above. For certain high-risk children between 8 and 19 months, including those with chronic lung disease, severe immune deficiency, cystic fibrosis with significant lung involvement, or American Indian and Alaska Native children, a second-season dose of 200 mg is recommended.
Long-Term Connection to Asthma
One concern parents often have is whether bronchiolitis means their child will develop asthma. A large meta-analysis found that children who had bronchiolitis before age two were about 2.5 times more likely to develop wheezing or asthma compared to children who did not. This association held up whether researchers looked at outcomes before age 10 or after age 10, suggesting it isn’t just a temporary tendency to wheeze that children grow out of quickly.
That said, a 2.5-fold increase in relative risk doesn’t mean asthma is inevitable. Most children who have bronchiolitis do not go on to develop asthma. Researchers still aren’t certain whether the bronchiolitis itself damages developing airways in a way that predisposes them to asthma, or whether children who get severe bronchiolitis already had an underlying tendency toward reactive airways. Either way, it’s worth mentioning to your child’s doctor at future visits if wheezing or persistent coughing recurs, so it can be evaluated in context.