RSV (respiratory syncytial virus) attacks the tiny airways in a baby’s lungs, causing inflammation, excess mucus, and breathing difficulty that can range from a mild cold to a serious illness requiring hospitalization. Nearly all children catch RSV by age two, but infants under 12 months are the most vulnerable. During the 2024–25 season, hospitalization rates for babies under one year reached roughly 1,050 to 1,117 per 100,000, making RSV one of the leading causes of infant hospitalization in the United States.
How RSV Damages Infant Airways
A baby’s airways are already narrow, some no wider than a pencil tip. RSV targets the cells lining those airways in a way that makes breathing progressively harder through several overlapping mechanisms.
First, the virus infects ciliated cells, the tiny hair-like structures responsible for sweeping mucus out of the lungs. Once infected, these cells round up, detach from the airway lining, and die. That means the natural cleaning system shuts down. At the same time, RSV infects deeper cells in the airway wall, which then transform into mucus-producing cells. The result is a double problem: the lungs produce far more mucus than normal while losing the ability to clear it. Thick mucus plugs accumulate in the smallest airways (the bronchioles), partially or fully blocking airflow.
On top of that, the airway walls swell with inflammation and fluid. When you combine swollen tissue, dead cell debris, and sticky mucus plugs inside airways that were already tiny, it’s easy to see why babies struggle to breathe. The exposed nerve fibers from damaged airway lining also trigger a persistent cough reflex. In older children and adults, these same airways are large enough that the swelling barely registers. In a baby, even a small amount of obstruction can make a real difference.
What It Looks Like Day by Day
RSV typically starts like a common cold. For the first two or three days, you’ll notice a runny nose, mild cough, and possibly a low fever. Many parents assume it’s nothing serious at this stage, and in many cases they’re right.
The tricky part is that RSV often worsens between days three and five. As the virus moves from the nose and upper throat down into the smaller airways, the cough deepens, breathing becomes faster, and you may hear wheezing. This is the window when most babies who need medical attention start showing concerning signs. Babies who are going to recover on their own generally start improving around days five through seven, with the cough lingering for up to two weeks. The full illness, from first sniffle to resolution, typically runs one to two weeks.
Signs of Serious Breathing Trouble
Most babies with RSV will be uncomfortable but fine. The ones who need urgent care show specific physical signs that are worth knowing. A breathing rate above 60 breaths per minute is a red flag. You can count this by watching your baby’s chest rise and fall for 30 seconds, then doubling the number.
Other warning signs include nasal flaring (the nostrils spreading wide with each breath), retractions (the skin pulling inward between the ribs, below the ribcage, or at the base of the throat with each inhale), and grunting. Grunting is a short, low sound at the end of each breath, and it means the baby is working hard to keep their airways open. Bluish color around the lips or fingertips signals that oxygen levels have dropped significantly. Difficulty feeding is another important sign, because babies who are breathing that hard often can’t coordinate sucking and swallowing at the same time.
Bronchiolitis vs. Pneumonia
RSV causes two main types of lower respiratory illness in babies. Bronchiolitis is the more common one, where inflammation and mucus concentrate in the smallest airways. This is what produces the characteristic wheezing and rapid breathing. Pneumonia occurs when the infection reaches the air sacs deeper in the lungs, filling them with fluid and inflammatory cells. Some babies develop both at the same time. Pneumonia tends to cause higher fevers and can look more systemically unwell, while bronchiolitis is more about the audible breathing difficulty. Both can require hospitalization depending on severity.
Which Babies Are Most at Risk
Premature babies face the highest risk because their airways are even smaller and their immune systems less developed. Babies born before 29 weeks of gestation are especially vulnerable. Infants with congenital heart defects or chronic lung disease also have a harder time compensating when their airways become obstructed. Babies under six months old are at greater risk than older infants simply because of the size of their airways and the immaturity of their immune response.
That said, the majority of babies hospitalized for RSV were previously healthy. Prematurity and underlying conditions increase the odds of severe disease, but they don’t account for most cases.
What Happens in the Hospital
There is no antiviral medication that treats RSV. Hospital care focuses entirely on keeping the baby stable while their immune system fights off the virus. The main interventions are straightforward: supplemental oxygen if levels drop too low, gentle nasal suctioning to clear mucus from the nose (using a bulb syringe or a parent-operated nasal aspirator with saline drops), and fluids through an IV or feeding tube if the baby can’t eat well enough on their own.
Deep suctioning, where a catheter is inserted far into the airway, is no longer recommended. Studies show it causes irritation and can actually extend hospital stays. Gentle surface-level suctioning of the nasal passages is the current standard. Most hospitalized babies improve within a few days with this type of supportive care and go home once they can breathe comfortably and feed on their own.
The Link to Childhood Asthma
RSV’s effects don’t always end when the infection clears. Research from the National Institutes of Health found that infants who avoided RSV infection during their first year of life had a 26% lower risk of developing asthma by age five. Among children who had RSV in infancy, 21% developed asthma by five, compared to 16% of those who didn’t. The researchers estimated that roughly 15% of early childhood asthma cases could be prevented if RSV infection during infancy were avoided entirely.
The likely explanation is that RSV-related damage and inflammation in developing airways can alter how those airways grow and respond to irritants for years afterward. This doesn’t mean every baby who gets RSV will develop asthma, but it does mean the virus can leave a lasting footprint on lung health.
Prevention With Nirsevimab
A monoclonal antibody called nirsevimab (sold as Beyfortus) is now available as a single injection for infants entering their first RSV season. It works by giving babies ready-made antibodies that block the virus from infecting airway cells. A large study from the 2024–25 RSV season found it was 77% effective at preventing RSV-related hospitalization. The peak daily hospitalization rate among treated infants was roughly 3 per 100,000, compared to nearly 14 per 100,000 among untreated infants.
The injection is typically given shortly after birth for babies born during RSV season (generally fall through early spring) or before the season starts for babies born in the months prior. Protection lasts about five months, which is enough to cover the peak risk window. For babies at high risk due to prematurity or heart or lung conditions, this prevention option is especially significant given how much more dangerous severe RSV can be for them.