Respiratory syncytial virus (RSV), also called human respiratory syncytial virus (hRSV) and human orthopneumovirus, is a common, contagious virus that causes infections of the respiratory tract. It is a negative-sense, single-stranded RNA virus. Its name is derived from the large cells known as syncytia that form when infected cells fuse.

RSV is the single most common cause of respiratory hospitalization in infants, and reinfection remains common in later life: it is a notable pathogen in all age groups. Infection rates are typically higher during the cold winter months, causing bronchiolitis in infants, common colds in adults, and more serious respiratory illnesses such as pneumonia in the elderly and immunocompromised.

RSV can cause outbreaks both in the community and in hospital settings. Following initial infection via the eyes or nose, the virus infects the epithelial cells of the upper and lower airway, causing inflammation, cell damage, and airway obstruction. A variety of methods are available for viral detection and diagnosis of RSV including antigen testing, molecular testing, and viral culture.

Treatment for severe illness is primarily supportive, including oxygen therapy and more advanced breathing support with CPAP or nasal high flow oxygen, as required. In cases of severe respiratory failure, intubation and mechanical ventilation may be required. Ribavirin is the only antiviral medication licensed for the treatment of RSV in children, although its use remains controversial.