Bronchiolitis is a common lung infection that causes swelling and mucus buildup in the smallest airways of a baby’s lungs. It mostly affects children under two years old and is the leading reason infants are hospitalized during the winter months. The illness typically starts like an ordinary cold before progressing to coughing, wheezing, and difficulty breathing over the course of several days.
What Happens Inside Your Baby’s Lungs
Your baby’s lungs contain thousands of tiny tubes called bronchioles that carry air to the parts of the lung where oxygen enters the blood. When a virus infects these tiny airways, the lining begins to swell and break down within 18 to 24 hours. The cells that normally sweep mucus out of the airways are damaged and replaced by cells that can’t do that job, while other cells ramp up mucus production. The result is airways that are simultaneously narrower from swelling and clogged with mucus and cellular debris.
In an adult, this might cause mild discomfort. In an infant, it can cause real breathing trouble. A baby’s bronchioles are already extremely small, so even a small amount of swelling can significantly reduce airflow. Air gets trapped behind the blockages, some portions of the lung may partially collapse, and the baby has to work much harder to breathe.
Which Viruses Cause It
Respiratory syncytial virus (RSV) is the most common cause of bronchiolitis by a wide margin. Nearly all children are infected with RSV at least once by age two, though not all of them develop bronchiolitis. Other viruses can trigger the same illness, including rhinovirus (the common cold virus), human metapneumovirus, influenza, and adenovirus. The specific virus rarely changes how the illness is treated, but RSV-caused cases are associated with a higher risk of recurrent wheezing later in childhood.
Symptoms and How They Progress
Bronchiolitis almost always begins with ordinary cold symptoms: a runny nose, mild cough, and sometimes a low fever. Over the next two to three days, the infection moves deeper into the lungs. That’s when parents typically notice the breathing changes that set bronchiolitis apart from a regular cold.
Signs of worsening illness include:
- Wheezing: a high-pitched whistling sound when your baby exhales
- Fast breathing: more than 60 breaths per minute in a newborn is considered rapid
- Retractions: the skin between or below the ribs visibly pulls inward with each breath, a sign your baby is working hard to get air
- Nasal flaring: the nostrils widen with each breath
- Poor feeding: babies who are struggling to breathe often can’t coordinate sucking and swallowing
- Pauses in breathing: especially in premature babies and those under two months old
Symptoms generally peak around days three to five of the illness, then gradually improve. A lingering cough can last two to three weeks even after the worst is over. The illness usually resolves on its own within seven to ten days for most babies.
Which Babies Are at Higher Risk
Babies younger than three months face the highest risk of severe bronchiolitis because their lungs and immune systems are still developing. Premature infants are particularly vulnerable since their airways are even smaller and their immune defenses less mature.
Other factors that increase the risk of a more serious course include having a congenital heart condition, chronic lung disease, a weakened immune system, or exposure to tobacco smoke. Babies who attend daycare or have older siblings in school are more likely to catch the viruses that cause bronchiolitis simply because of greater exposure, though this doesn’t necessarily mean their illness will be more severe.
How Bronchiolitis Is Treated
There is no medication that cures bronchiolitis or meaningfully shortens it. This surprises many parents, but the evidence is clear: bronchodilators (the type of inhaler used for asthma), steroids, and antibiotics do not help in routine cases. The American Academy of Pediatrics specifically recommends against all three. Bronchodilators may briefly improve how a baby sounds but do not reduce hospitalizations or speed recovery. Antibiotics fight bacteria, not viruses, and should only be used if a separate bacterial infection is present.
Treatment is supportive, meaning the goal is to keep your baby comfortable and well-hydrated while the immune system clears the virus.
What You Can Do at Home
Keeping your baby’s nose clear is one of the most helpful things you can do. Use saline nose drops to thin the mucus, then gently suction it out with a bulb syringe. Squeeze the bulb before placing the rubber tip in one nostril, then slowly release it to draw mucus out. This works best in babies younger than six months. You may need to do this before feedings and before sleep.
Hydration is critical. Babies with bronchiolitis often feed more slowly or refuse feeds because breathing takes so much effort. Offer smaller, more frequent feedings. Some babies prefer clear liquids over milk or formula during the worst days. Watch for signs of dehydration: fewer wet diapers, dry mouth, crying without tears, or unusual sleepiness.
A cool-mist humidifier in the baby’s room can help keep airways moist. Keeping your baby slightly upright (such as in a reclined position while supervised) may also ease breathing, though always follow safe sleep guidelines when putting your baby down for the night.
When Babies Need Hospital Care
Some babies need more support than home care can provide. In the hospital, treatment typically involves supplemental oxygen if blood oxygen levels drop, and IV fluids if the baby is too exhausted or struggling too much to feed. In severe cases, a baby may need help breathing through a machine that delivers a steady stream of air to keep the airways open. Most hospitalized babies stay for two to four days.
Seek emergency care if your baby shows bluish color around the lips or fingertips, is breathing very rapidly with visible retractions or grunting, has pauses in breathing, or becomes lethargic and difficult to rouse.
The Link to Wheezing and Asthma
One concern parents often have is whether bronchiolitis will lead to asthma. A six-year follow-up study of infants hospitalized with bronchiolitis found that about 32% developed recurrent wheezing and 12.5% were eventually diagnosed with asthma. Babies whose bronchiolitis was caused by RSV had higher rates of later wheezing, as did babies with a history of eczema. Having eczema roughly doubled the rate of recurrent wheezing compared to babies without it.
This does not mean bronchiolitis causes asthma. It may be that the same underlying immune tendencies that make a baby react strongly to RSV also predispose them to wheezing and allergic conditions. Still, if your baby has had bronchiolitis and you notice repeated episodes of wheezing in the following years, it’s worth discussing with your pediatrician.
Prevention: RSV Immunization
Since RSV is the primary cause of bronchiolitis, preventing RSV infection is the most effective way to reduce your baby’s risk. A preventive antibody called nirsevimab (brand name Beyfortus) is now recommended for infants younger than eight months who are born during or entering their first RSV season, which runs roughly from October through March in most of the continental United States. It’s given as a single injection and is recommended if the mother did not receive an RSV vaccine during pregnancy, if her vaccination status is unknown, or if the baby was born within 14 days of maternal vaccination.
Real-world data show nirsevimab is at least 70% effective at preventing RSV hospitalizations, and clinical trials found about 80% effectiveness against RSV-related hospitalization. For 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 dose is recommended before their second RSV season as well.
Beyond immunization, basic hygiene measures help: wash hands before touching your baby, keep sick contacts away, avoid crowded indoor spaces during peak RSV season, and never smoke around your child.