Respiratory Syncytial Virus is a common respiratory virus that can infect people of all ages but is particularly important because it can cause bronchiolitis and pneumonia in infants and severe respiratory disease in some older adults and people with underlying medical conditions. The virus is an enveloped, negative-sense single-stranded RNA virus in the Pneumoviridae family and has historically been recognised as an important cause of lower respiratory tract infection in young children.
In many adults and older children, infection resembles a Common Cold, but infants can develop rapid breathing, wheezing, difficulty feeding, dehydration and respiratory distress. The term “syncytial” refers to the formation of multinucleated cellular structures called syncytia, produced when infected cells fuse with neighbouring cells.
Respiratory Syncytial Virus has two major antigenic subgroups, commonly designated RSV A and RSV B, with multiple circulating genetic lineages within them. The viral genome encodes structural and regulatory proteins involved in attachment, fusion, replication and evasion of host immune responses. The fusion protein is particularly important biologically and clinically because it mediates viral entry and cell-to-cell fusion and has become a major target for preventive interventions.
RSV infection does not produce lifelong sterilising immunity, so reinfections are common throughout life. The first infection in early childhood can be more severe because infants have small airways and relatively limited physiological reserve, while subsequent infections often cause milder upper-respiratory disease, although severe reinfection remains possible in vulnerable adults.
RSV spreads primarily through respiratory secretions and close contact, including exposure to infectious droplets or particles and contact with contaminated hands and surfaces followed by contact with the eyes, nose or mouth. It is highly transmissible in households, childcare environments, hospitals and other settings where people are in close proximity. The incubation period is generally several days, and infected people can transmit the virus for a period extending beyond the appearance of symptoms.
Infants and young children can sometimes shed virus for substantially longer than healthy adults, particularly when immune function is impaired. Seasonal circulation patterns vary geographically, but RSV often produces marked annual waves of infection. Public-health surveillance is therefore useful for planning healthcare capacity, preventive interventions and interpretation of respiratory-disease activity.
In young infants, RSV can infect the small airways and produce bronchiolitis, an inflammatory illness characterised by airway swelling, mucus production and impaired airflow. Because infant airways are narrow, relatively modest amounts of swelling and mucus can have a disproportionately large effect on resistance to airflow. Clinical features can include rapid breathing, wheezing or crackles, feeding difficulty, irritability and reduced oxygen saturation. Very young infants may instead show non-specific signs such as lethargy, poor feeding or pauses in breathing. Most children recover with supportive care, but some require hospitalisation for oxygen, hydration or respiratory support. Risk of severe disease is higher with younger age and certain medical factors, including prematurity, chronic lung disease, congenital heart disease and impaired immune function.
In older adults, RSV can produce a respiratory illness resembling Influenza or other viral infections and can exacerbate chronic conditions such as chronic obstructive pulmonary disease or heart disease. Diagnosis may involve molecular testing, especially in hospitals, outbreaks or high-risk patients, but testing is not always necessary for mild community illness. There is no general antiviral treatment routinely required for otherwise healthy people with uncomplicated RSV infection, so care is primarily supportive. Severe disease may require supplemental oxygen, intravenous or enteral hydration and, in extreme cases, mechanical respiratory support. Antibiotics do not treat RSV itself, although bacterial infection can coexist or develop secondarily and would require separate treatment when clinically established.
Prevention has increasingly included targeted immunisation and antibody-based protection. Vaccines are available for certain adult age groups and for pregnancy in relevant jurisdictions, while long-acting monoclonal antibody protection can be used to protect eligible infants during their first RSV season. These interventions work by providing immune protection before or around the period of greatest risk rather than treating established infection. Eligibility, timing and product availability depend on national guidance and individual circumstances.
Other preventive measures include reducing exposure of vulnerable infants to people with respiratory symptoms, improving hand hygiene and cleaning frequently touched surfaces. Because RSV circulates widely and reinfection occurs, prevention strategies are designed primarily to reduce severe disease and protect high-risk groups rather than to eliminate every infection.
RSV has become an important focus of respiratory epidemiology because seasonal waves can produce substantial demand for paediatric and adult healthcare services. Severe bronchiolitis can require hospital beds, oxygen equipment and respiratory support, particularly when large numbers of infants become infected simultaneously. The burden is not limited to infants, because older adults can experience pneumonia, exacerbations of chronic lung disease and cardiovascular complications.
RSV also illustrates why age is a major determinant of infectious-disease presentation: the same virus may produce mild nasal symptoms in one person and serious lower-airway disease in another. Surveillance systems monitor RSV activity alongside other respiratory viruses because overlapping seasonal epidemics can complicate diagnosis, hospital planning and interpretation of community respiratory illness.
A common misconception is that RSV is exclusively a childhood infection or that an RSV diagnosis automatically means severe disease. In reality, RSV infects people repeatedly across the lifespan, and severity depends on age, immune status, airway anatomy and underlying health. Another misconception is that wheezing necessarily means asthma; RSV-associated bronchiolitis can produce wheezing through acute small-airway inflammation, although recurrent wheezing after respiratory infections may require separate assessment.
RSV should also not be treated as synonymous with bronchiolitis because bronchiolitis is a clinical syndrome that can have causes other than RSV. Its clinical significance lies in the combination of very high exposure frequency, repeated infections, substantial infant morbidity, meaningful disease in older adults and the increasing ability to prevent severe illness through targeted immunological interventions.
