Respiratory syncytial virus (RSV) is a common respiratory virus that can cause a wide range of symptoms, from mild to severe. The virus is a leading cause of lower respiratory tract infections in infants and young children, which can lead to hospitalization or death.   In healthy children and adults, RSV typically causes mild symptoms such as a runny nose, cough, and low-grade fever. These symptoms usually resolve on their own within a week or two. However, in certain populations, such as infants and older adults with weakened immune systems, RSV can lead to much more serious health problems.   Infants and young children are at a higher risk for developing bronchiolitis after an RSV infection, which is inflammation of the small airways in the lungs. This can cause difficulty breathing, wheezing, and a persistent cough. In severe cases, infants with bronchiolitis may require hospitalization and treatment with oxygen.   Older adults with weakened immune systems, or those with chronic lung or heart disease, are at risk of developing pneumonia, which is a lung infection that can cause fever, cough, and difficulty breathing. Pneumonia caused by RSV can be very serious and lead to hospitalization, and in some cases, death.