Bronchiolitis is a common lung infection that primarily affects the smallest airways in young children’s lungs, causing them to swell and fill with mucus. This condition, most often triggered by viruses, can turn a simple cold into a frightening struggle to breathe, making it the leading reason infants end up in hospital during their first year of life.

Understanding the Scope of Bronchiolitis

Bronchiolitis stands as the most common lower respiratory tract infection among children younger than two years of age. During the first year of life, approximately 11 to 15 percent of infants will develop this condition^[2]. The infection is serious enough that it results in at least five hospitalizations for every 1,000 children under two years old^[2]. While most cases remain mild and manageable at home, the condition can escalate into respiratory failure in some infants, particularly those with underlying health conditions^[2].

The disease follows a distinct seasonal pattern, appearing most commonly during autumn and winter months, though sporadic cases can occur throughout the year^[2]. In the northern hemisphere, the majority of cases cluster between December and February, while in the southern hemisphere, peak season runs from May to July^[8]. This predictable timing allows healthcare systems to prepare for the annual surge in cases, though recent years have seen the illness extend into what appears to be a year-round presence^[22].

Each year, approximately 150 million children worldwide receive a diagnosis of bronchiolitis^[8]. The condition affects infants most severely, particularly those between two and six months of age^[7]. This vulnerability stems from the fact that babies have much smaller airways than older children, meaning even minor swelling and mucus buildup can significantly block airflow and make breathing difficult^[1].

What Causes Bronchiolitis

A virus causes bronchiolitis in nearly all cases. The condition is not a bacterial infection, which means antibiotics (medicines designed to kill bacteria) will not help treat it^[9]. Instead, the illness results from viral invasion of the tiny airways called bronchioles, which are the smallest passageways in the lungs that lead to air sacs where oxygen enters the blood^[1].

Respiratory syncytial virus, commonly known as RSV, stands as the most frequent culprit behind bronchiolitis. This virus accounts for approximately 75 percent of cases in children younger than two years who require hospitalization^[7]. RSV belongs to a family of viruses that cause respiratory infections, and it exists in two main subtypes, A and B, based on structural variations. Subtype A typically causes more severe infections, and one subtype or the other usually dominates during a given season, creating what healthcare providers recognize as “good” and “bad” years for RSV disease^[7].

However, RSV is far from the only viral cause of bronchiolitis. Other viruses that can trigger the condition include human rhinovirus (the virus that causes the common cold), parainfluenza virus, human metapneumovirus, adenovirus, coronavirus, influenza virus, human bocavirus, and even SARS-CoV-2, the virus responsible for COVID-19^[2][10]. In about 30 percent of hospitalized cases, laboratory testing detects two or more viruses present simultaneously, especially when using modern molecular-based diagnostic methods^[2][7].

Risk Factors That Increase Vulnerability

While any infant can develop bronchiolitis, certain groups face higher risks of severe infection requiring hospitalization or intensive medical care. Age plays a crucial role, with children younger than two years being most susceptible, and those under three months facing particularly elevated risks^[2][4]. The condition typically affects infants between one and six months of age most severely^[7].

Premature birth significantly increases vulnerability to severe bronchiolitis. Infants born before 32 to 34 weeks of pregnancy face heightened risk because their lungs and immune systems have had less time to develop fully^[2][4]. Similarly, babies with low birth weight carry elevated risk of more serious illness^[2].

Chronic medical conditions make bronchiolitis more dangerous for affected infants. Those with congenital heart disease (heart problems present from birth) face greater risk of severe infection, as do children with chronic lung diseases such as bronchopulmonary dysplasia (a lung condition that sometimes develops in premature babies who receive oxygen therapy)^[2][4]. Infants with neuromuscular disease, which affects nerves and muscles, or immunodeficiency disorders that weaken the immune system also belong to high-risk groups^[2].

Environmental factors play an important role as well. Children exposed to parental smoking or secondhand smoke have higher rates of bronchiolitis and more severe cases^[2][6]. Living in crowded environments or attending daycare increases exposure risk because the viruses spread easily in group settings where children have close contact^[2][10]. Low socioeconomic status correlates with increased bronchiolitis risk, likely due to factors including crowded living conditions, limited access to healthcare, and higher rates of exposure to cigarette smoke^[2].

Aboriginal and Torres Strait Islander babies in Australia face higher risks of serious bronchiolitis compared to other populations^[3]. Additionally, infants with airway abnormalities present from birth have elevated susceptibility to the infection^[2].

Recognizing the Symptoms

Bronchiolitis typically begins with symptoms that closely resemble a common cold, making early recognition challenging for parents. In the first few days, affected children develop a runny or stuffy nose, sneezing, mild cough, and sometimes a slight fever^[1][3]. These initial signs appear innocuous, similar to countless other viral infections children experience.

After one or two days, however, the illness often progresses as the virus affects the small airways more deeply. The cough typically worsens and becomes more prominent^[1][3]. Children begin breathing faster than normal, a condition called tachypnea^[4]. Parents may hear a high-pitched whistling sound when the child breathes out, known as wheezing^[1][3]. The cough may become raspy and crackly, and some children cough so hard they vomit, with the vomit containing thick mucus^[22].

As breathing becomes more difficult, infants work harder to get air into their lungs. Observable signs of this struggle include retractions, where the skin between the ribs, below the rib cage, or around the neck appears to suck inward with each breath^[3][4][10]. The nostrils may flare outward as the child attempts to draw in more air^[3][10]. Younger babies may bob their heads with each breath^[3]. Some infants make grunting noises when breathing^[4][10].

The difficulty breathing makes feeding problematic because infants cannot coordinate sucking, swallowing, and breathing simultaneously when they are struggling for air. This leads to poor feeding and increases the risk of dehydration (dangerous loss of body fluids)^[1][3]. Affected babies may become fussy, restless, irritable, or unusually tired^[3][10].

Symptoms typically peak around the second or third day of illness and are usually worst during the first five days^[3][6]. The acute illness generally lasts seven to ten days, though some children remain unwell for one to two weeks^[1][3][9]. The cough often persists longer, sometimes continuing for up to four weeks even after other symptoms resolve^[3].

Many infants with bronchiolitis also develop an ear infection called otitis media, with more than half of children aged three to eighteen months experiencing this complication^[1][7].

Prevention Strategies

Preventing bronchiolitis centers on reducing exposure to the viruses that cause it and protecting vulnerable infants through immunization when appropriate. Since the viruses spread like common cold viruses, similar preventive measures apply. Avoiding contact with children or adults who have respiratory infections provides the first line of defense^[6]. When someone in the household has bronchiolitis or an upper respiratory infection, separating them from infants when possible reduces transmission risk. Placing an ill child in a separate room to sleep can help^[6].

Hand-washing stands as one of the most effective prevention measures. Washing hands frequently removes germs and helps prevent their spread to infants when adults or siblings touch the baby or objects the baby might touch^[6]. If a child has bronchiolitis, keeping them home from school or daycare until they recover prevents the infection from spreading to other children^[6].

Avoiding exposure to tobacco smoke protects children from bronchiolitis in multiple ways. Parents should not smoke or use other tobacco products around their children, as secondhand smoke irritates the mucous membranes in the nose, sinuses, and lungs, increasing both the risk of respiratory infections and the severity of bronchiolitis if it develops^[6]. Smoking or vaping around babies can make bronchiolitis worse once they have it^[3].

New immunization options offer promising protection against bronchiolitis caused by RSV. In early 2025, the Australian Government launched a free program called the RSV Mother and Infant Protection Program, which includes vaccination for pregnant women and immunization for babies^[3]. Both products have proven very effective at preventing babies from requiring hospitalization with RSV^[3].

For high-risk infants, including those born prematurely or with chronic lung or heart conditions, preventive medications may be recommended. Options include RSV immunoglobulin or the anti-RSV monoclonal antibody called palivizumab, which can decrease disease severity in vulnerable populations^[6][12]. Parents of babies in high-risk groups should discuss these preventive options with their healthcare provider.

How the Body Changes During Bronchiolitis

Understanding what happens inside the body during bronchiolitis helps explain why affected children struggle so much with breathing. The process begins when virus particles enter the respiratory system and infect the epithelial cells lining the airways. These epithelial cells normally form a protective barrier and help move mucus upward and out of the lungs through tiny hair-like structures called cilia^[2].

Once infected, the epithelial cells become damaged, and many die. The cilia stop functioning properly, losing their ability to clear mucus effectively^[2]. Meanwhile, the body’s immune system responds to the viral invasion by triggering an inflammatory reaction in the airways. This inflammation causes the walls of the bronchioles to swell with fluid, a condition called edema^[4][7].

As infected cells die and the immune response continues, debris accumulates in the airways. The airways also produce excessive amounts of thick mucus in response to the infection and inflammation^[2][3]. This combination of swollen airway walls, dead cells, inflammatory debris, and thick mucus significantly narrows the tiny bronchioles, obstructing the flow of air into and out of the lungs^[7][8].

The accumulation of mucus creates particular problems because babies younger than about nine months breathe primarily through their noses rather than their mouths^[22]. When thick mucus blocks their nasal passages and they cannot yet blow their noses effectively, they experience significant breathing difficulties^[22].

The airway obstruction affects lung mechanics in several ways. It becomes harder to move air in and out of the lungs, which is why children breathe faster and work harder, using extra respiratory muscles and creating the visible retractions described earlier. The lungs also lose some of their normal elasticity and compliance, meaning they become stiffer and harder to expand^[2].

In severe cases, the combination of blocked airways and impaired lung function leads to inadequate oxygen reaching the bloodstream. Blood oxygen levels drop below normal, a condition called hypoxemia^[8][9]. This explains why some children develop bluish discoloration of their skin, lips, or nail beds, and why supplemental oxygen becomes necessary for treatment in hospitalized patients.

The virus can continue shedding in nasal secretions for six to twenty-one days after symptoms develop, meaning affected children remain potentially contagious for weeks^[7]. The incubation period (time from exposure to symptom onset) is two to five days^[7].