They spent Christmas that year at the hospital, where Andre was admitted with a blood oxygen level of 70 percent. It should be close to 100 percent.

Like many parents, Ms. Salahshour was unfamiliar with R.S.V. “I remember just being in the corner, kind of hyperventilating a little bit,” she said.

In otherwise healthy patients, R.S.V. can usually be treated at home. Children who have been infected with the virus produce antibodies that help reduce its severity if they become reinfected. But R.S.V. can turn into acute lower respiratory infections such as bronchiolitis, a viral respiratory illness that is the most common cause of hospitalization in infants, Dr. Wiener said. It can also lead to pneumonia.

Each year, on average, the virus results in more than 57,000 hospitalizations among children younger than 5, according to the Centers for Disease Control.

Ms. Martin’s son was discharged after staying overnight at the hospital, where he received fluids and oxygen. “When I left it was literally a room full of tiny kids coughing, coughing, coughing,” she said.

When to worry

“When we get concerned is when we’re seeing that kids are having more trouble breathing and they’re not feeding well,” said Dr. Robert Adler, chief medical officer of the Children’s Hospital Los Angeles Health System.

Image Kate Lacovara-Green, 15 months, with her father Sam Green on Dec. 7 in the Pediatric Intensive Care Unit at the New York Presbyterian Hospital-Cornell Medical Center on day three of her seven-day hospital stay. Credit... via Chris Lacovara

Other worrisome symptoms include dehydration, fever, fussiness, signs of dehydration or distress, and lethargy.