When little Addy Sylvester started to have trouble breathing in February 2016, her parents took her to the doctor. They were given a pamphlet on respiratory syncytial virus, or RSV, and told to keep a close eye on Addy. Eighteen hours later, their sweet-natured, happy and otherwise healthy baby was gone. She hadn’t yet reached her first birthday. Addy’s mom, Sarah Sylvester of Gahanna, Ohio, is now focused on helping researchers at Nationwide Children’s Hospital learn more about RSV, the No. 1 cause of infant hospitalizations in the United States. She also wants to alert other parents to its risk. “One thing that stuck for me from the very beginning was just how surprised I was that one, something like this could happen to a healthy baby, and two, my lack of awareness as an educated adult,” Sylvester said. “I had never heard of it and was grossly unaware of the severity of it as well.” Though it is rare for an infant in the United States to die from RSV, the virus is the second-leading cause of death, behind malaria, among children younger than 1 worldwide, said Dr. Octavio Ramilo, chief of infectious diseases at Nationwide Children’s. About 50 to 60 percent of children will have contracted RSV by age 1 and almost 100 percent will have encountered it by age 2. Even so, most parents have never heard of it. At Nationwide Children’s, it is the most common reason that parents bring infants to a physician, emergency department or urgent care, Ramilo said. About 600 to 700 babies are hospitalized at the facility because of the illness each year, with 150 to 200 ending up in the intensive-care unit in need of breathing support and IV fluids. Though some conditions, such as prematurity, chronic lung disease or congenital heart disease, can make babies more susceptible, about 80 percent of those hospitalized with RSV are otherwise healthy, Ramilo said. Researchers at Nationwide Children’s seek to understand why some children get severe lung infections from RSV while others only exhibit mild cold symptoms and a fever. They also are among the scientists at various hospitals, foundations and pharmaceutical companies looking to develop a vaccine for RSV as well as better treatment options, Ramilo said. “We need to think about these babies, and we need to do better,” he said. “The perception is that babies get over things and babies have to get through things in life, but this is a serious one.” RSV is most commonly seen from November to April, and it can be carried home to infants by older siblings who pick up the virus in school. Symptoms include difficulty breathing and inability to take a bottle. Those symptoms can linger for those who become severely ill, and such children might be susceptible to recurrent infections for years, Ramilo said. Parents, Ramilo said, should take precautions against exposing infants to people with colds, who could harbor RSV. “We need to keep young babies, especially in the first year of life, separate from siblings or family members who have respiratory symptoms,” he said. “You have to be overprotective of these young infants, especially now, between November and April — that’s when we have respiratory infections in the community.” Since Addy died in 2016, the Sylvesters have had another child, Michael. He’s almost 6 months old. The couple placed their son in a Nationwide Children’s research study and are working with the hospital to raise awareness about RSV. Parents, Sarah Sylvester said, should know that RSV is something that children most likely will get. “Educate yourself, make sure you’re talking to your doctors about it when you have checkups so you know what to look out for,” she said. “And don’t underestimate it.” — JoAnne Viviano writes about health and medicine for The Columbus (Ohio) Dispatch. Email her at firstname.lastname@example.org or follow her on Twitter at @JoAnneViviano.