Respiratory syncytial virus infection, usually called RSV, is a lot like a bad cold. It causes the same symptoms. And like a cold, it is very common and very contagious. Most children have had it at least once by age 2. RSV is usually not something to worry about, but it can lead to pneumonia or other problems in some people, especially babies. So, it's important to monitor the patient and call a doctor if symptoms get worse.