In temperate climates there is an annual epidemic during the winter months. In tropical climates, infection is most common during the rainy season.

In the United States, 60% of infants are infected during their first RSV season,^[1] and nearly all children will have been infected with the virus by 2–3 years of age.^[1] Of those infected with RSV, 2–3% will develop bronchiolitis, necessitating hospitalization.^[2] Natural infection with RSV induces protective immunity which wanes over time—possibly more so than other respiratory viral infections—and thus people can be infected multiple times. Sometimes an infant can become symptomatically infected more than once, even within a single RSV season. Severe RSV infections have increasingly been found among elderly patients.

RSV is a negative-sense, single-stranded RNA virus of the family Paramyxoviridae, which includes common respiratory viruses such as those causing measles and mumps. RSV is a member of the paramyxovirus subfamily Pneumovirinae. Its name comes from the fact that F proteins on the surface of the virus cause the cell membranes on nearby cells to merge, forming syncytia.

For most people, RSV produces only mild symptoms, often indistinguishable from common colds and minor illnesses. The Centers for Disease Control consider RSV to be the "most common cause of bronchiolitis (inflammation of the small airways in the lung) and pneumonia in children under 1 year of age in the United States."^[3] For some children, RSV can cause bronchiolitis, leading to severe respiratory illness requiring hospitalization and, rarely, causing death. This is more likely to occur in patients that are immunocompromised or infants born prematurely. Other RSV symptoms common among infants include listlessness, poor or diminished appetite, and a possible fever.^[4]

Recurrent wheezing and asthma are more common among individuals who suffered severe RSV infection during the first few months of life than among controls;^[5] whether RSV infection sets up a process that leads to recurrent wheezing or whether those already predisposed to asthma are more likely to become severely ill with RSV has yet to be determined.

Symptoms of pneumonia in immuno-compromised patients such as in transplant patients and especially bone marrow transplant patients should be evaluated to rule out RSV infection. This can be done by means of PCR testing for RSV nucleic acids in peripheral blood samples if all other infectious processes have been ruled out or if it is highly suspicious for RSV such as a recent exposure to a known source of RSV infection.

In at least one case, RSV onset appears to have coincided with the onset of type 2 diabetes, and there may be a possible correlation.^{[6] [7]}

RSV spreads easily by direct contact, and can remain viable for a half an hour or more on hands or for up to 5 hours on countertops.^[10]