Pediatricians are warning parents to be on the lookout for respiratory syncytial virus, which is now entering the peak season.

RSV often starts out like a cold, with a sniffle, runny nose, and sometimes a low-grade fever. But as the virus moves into the lungs, kids start to have trouble breathing. Usually symptoms are at their worst on the third day of the illness.

Look for fast breathing, pushing the belly in and out, or flaring the nostrils — signs your baby is struggling to breathe. Some kids will stop breathing entirely for a few seconds, called apnea.

Other RSV symptoms include a lack of energy and poor feeding. “With a breastfeeding baby, it can be difficult to tell how much they’re eating,” said Janet Englund, MD, a professor of pediatric infectious diseases at the University of Washington and Seattle Children’s Hospital. “You want to see a wet diaper every 6 hours.”