Respiratory syncytial virus bronchiolitis is a lung infection affecting the tiniest airways in young children, most commonly caused by RSV. While many babies experience only mild cold-like symptoms, some develop serious breathing difficulties requiring hospital care, especially those under six months old.

Understanding the Disease and Who It Affects

Respiratory syncytial virus bronchiolitis occurs when a viral infection causes inflammation and swelling in the bronchioles, which are the smallest air passages deep inside the lungs. When these tiny tubes become inflamed and filled with mucus, air cannot flow freely, making it hard for a child to breathe properly. The most common virus responsible for this condition is respiratory syncytial virus, or RSV, though other viruses can also trigger bronchiolitis.^[1][2]

This condition primarily strikes infants and young children under two years of age. The reason young babies are so vulnerable is that their airways are naturally very small, so even a small amount of swelling can block airflow significantly. Additionally, their immune systems are still developing and haven’t yet learned how to fight off this particular virus effectively.^[2][3]

How Common Is This Infection?

Bronchiolitis caused by RSV is remarkably widespread among young children. Nearly all children become infected with RSV at least once before their second birthday, and about half of these children will contract it twice during their first two years of life. During the first year of life alone, the incidence of bronchiolitis ranges from 11% to 15% of all infants.^[1][4]

While most children recover at home without complications, a significant number require medical attention. In the United States, approximately 2% to 3% of infants younger than 12 months are hospitalized with RSV infection each year. This translates to more than 57,500 hospitalizations and 2.1 million outpatient visits associated with RSV infections annually in U.S. children younger than five years. The condition accounts for at least five hospitalizations for every 1,000 children younger than two years of age.^[2][4]

The illness follows a predictable seasonal pattern. In the United States, RSV infections occur most commonly between October and May, with cases peaking in December through March. During these winter months, emergency departments and pediatric hospitals experience surges in young patients struggling to breathe due to bronchiolitis.^[2][3]

How the Virus Spreads

RSV spreads remarkably easily from person to person, which explains why nearly every child eventually catches it. The virus travels through respiratory droplets that an infected person releases when coughing or sneezing. A child can become infected by having direct contact with someone who is sick, or by touching surfaces contaminated with the virus and then touching their own eyes, mouth, or nose.^[2][3]

The virus is remarkably hardy and can survive on hard surfaces like toys, countertops, and furniture for several hours. This durability makes places where many children gather—such as daycare centers, nurseries, and doctor’s waiting rooms—particularly high-risk environments for transmission. An infected person can spread the virus for three to eight days while they have symptoms, and sometimes they can pass it along even a day or two before symptoms appear.^[3][6]

Who Is at Highest Risk?

While any infant can develop bronchiolitis, certain groups face a much higher risk of severe illness requiring hospitalization. Infants younger than three months are particularly vulnerable, especially those born prematurely (before 32 to 34 weeks of pregnancy). Premature babies have smaller, less developed airways and immature immune systems, making them less able to handle respiratory infections.^[4][2]

Children with underlying medical conditions face elevated risks as well. These high-risk groups include those with chronic lung disease (such as bronchopulmonary dysplasia, a condition affecting premature infants), congenital heart disease, neuromuscular disorders, or weakened immune systems. Children with cystic fibrosis or those undergoing chemotherapy are also more susceptible to severe complications.^[4][6]

Additional risk factors that increase the likelihood of severe infection include low birth weight, exposure to tobacco smoke (especially if parents smoke), living in crowded conditions, and coming from lower socioeconomic populations. Male infants also appear to be at slightly higher risk than females.^[4]

Recognizing the Symptoms

The illness typically begins like an ordinary cold. Parents usually notice two to four days of upper respiratory symptoms that seem unremarkable: a runny or stuffy nose, mild fever, congestion, and perhaps some sneezing. During this early phase, many parents don’t realize their child has anything more serious than a common cold. Symptoms typically appear about four to six days after exposure to the virus.^[1][2]

However, after these initial cold-like symptoms, the infection can progress to affect the lower airways. This is when bronchiolitis becomes apparent. The child develops an increasing cough, and parents begin to hear wheezing—a high-pitched whistling or purring sound that occurs when the child breathes out. The child’s breathing becomes noticeably faster than normal, often exceeding 40 breaths per minute, and they may appear to be working hard to push air out of their lungs.^[2][4]

In very young infants, especially those under six months, the symptoms may look different. These tiny babies might become unusually irritable or lethargic, refuse to eat or drink, or experience frightening pauses in breathing called apnea. Some infants don’t develop a fever at all, which can make parents underestimate the severity of the illness.^[1]

Warning signs that indicate a child needs immediate medical attention include struggling with each breath, chest muscles and skin pulling inward with each breath (called retractions), nostrils flaring out when breathing, rapid or shallow breathing, pauses in breathing, a bluish color appearing around the lips or face, extreme difficulty feeding, or the child becoming unusually sleepy or unresponsive.^[1][6]

Preventing the Spread

Since no cure exists for viral bronchiolitis, prevention becomes extremely important. The most effective preventive measure is simple but crucial: frequent handwashing. Parents, caregivers, and anyone who comes into contact with infants should wash their hands thoroughly and often, especially after coughing, sneezing, or touching potentially contaminated surfaces.^[2]

People who are sick should stay away from young infants whenever possible. If someone in the household has cold symptoms, they should cover their nose and mouth with a tissue when coughing or sneezing and avoid kissing or closely handling the baby until they recover. Toys and surfaces that young children frequently touch should be cleaned regularly, as the virus can survive on these objects for hours.^[3]

For high-risk infants, medical prevention options exist. A medication called palivizumab is a monoclonal antibody that can help protect vulnerable babies. This preventive treatment is recommended for specific groups: infants born before 29 weeks of pregnancy, infants with chronic lung disease of prematurity, and infants and children with significant heart disease. The medication is given as a monthly injection during RSV season, typically from November through March, for up to five doses.^[2][8]

More recently, additional preventive options have become available. A vaccine can be given to pregnant women to help protect their babies after birth, and a newer immunization option called nirsevimab can be given to infants before or during their first RSV season. These tools represent significant advances in protecting the most vulnerable babies from severe illness.^[12]

What Happens Inside the Body

Understanding what bronchiolitis does inside the lungs helps explain why children become so sick. When RSV enters the respiratory tract, it infects the cells lining the airways, particularly targeting the epithelial cells in the bronchioles. The virus causes direct damage to these cells, leading to their death and destruction. This process is called necrosis.^[2][4]

As infected cells die and slough off, they mix with mucus to create thick plugs that block the tiny bronchioles. The body’s immune system responds to the infection by triggering inflammation, which causes the airway walls to swell. This combination of mucus plugs, dead cells, and swelling severely narrows the air passages.^[4]

The narrowed, blocked airways create a cascade of breathing problems. Air becomes trapped in parts of the lung, causing hyperinflation (over-expansion of the lungs), while other areas collapse, a condition called atelectasis. The child must work much harder to move air in and out of their lungs. Oxygen levels in the blood may drop because blocked airways prevent efficient gas exchange. Meanwhile, the effort of breathing becomes exhausting, and in severe cases, a child can develop respiratory fatigue where their breathing muscles simply cannot keep up with the demands.^[4]

The inflammatory response also affects lung compliance, meaning the lungs become stiffer and harder to expand. This stiffness, combined with airway obstruction, creates the characteristic wheezing sound and rapid breathing pattern seen in bronchiolitis. The body tries to compensate by breathing faster and working harder, but this compensation can only go so far before a child needs medical support.^[4]