Dr. Sean O'Leary, an infectious diseases physician at Children's Hospital Colorado, said he is seeing a "big uptick" in RSV cases. Other viruses, normally seen in the winter, are also landing children in the hospital this summer, including enteroviruses and parainfluenza 3, which causes croup. "We're seeing cases of bronchiolitis, which is what RSV usually causes, but we're seeing it with the other viruses," O'Leary said.