RSV is the most common respiratory pathogen in infants and young children. It has infected nearly all infants by the age of two years. Seasonal outbreaks of acuterespiratory illness occur each year, on a schedule that is somewhat predictable in each region. The season typically begins in the fall and runs into the spring.

RSV is spread easily by physical contact. Touching, kissing, and shaking hands with an infected person can spread RSV. Transmission is usually by contact with contaminated secretions, which may involve tiny droplets or objects that droplets have touched. RSV can live for half an hour or more on hands. The virus can also live up to five hours on countertops and for several hours on used tissues. RSV often spreads very rapidly in crowded households and day care centers.

In infants and young children, RSV can cause pneumonia, bronchiolitis (inflammation of the small airways of the lungs), and tracheobronchitis (croup). In healthy adults and older children, RSV is usually a mild respiratory illness. Although studies have shown that people produce antibody against the virus, infections continue to occur in people of all ages.

Each year up to 125,000 infants are hospitalized due to severe RSV disease; and about 1-2% of these infants die. Infants born prematurely, those with chronic lung disease, those who are immunocompromised, and those with certain forms of heart disease are at increased risk for severe RSV disease. Those who are exposed to tobacco smoke, who attend daycare, who live in crowded conditions, or have school-age siblings are also at higher risk.