A virus that usually sickens children during cold weather months has surprised doctors and put many babies in the hospital in the United States. The virus is called RSV, or respiratory syncytial virus. It is a common sickness that is like the common cold. But the virus can also cause severe sickness in babies and old people. Cases of RSV dropped sharply last year, when people mostly stayed at home and avoided busy areas during the COVID-19 health crisis. But RSV cases began increasing as pandemic restrictions eased. “I’ve never seen anything like this before,” said Dr. Kate Dutkiewicz. She is medical director at Beacon Children’s Hospital in South Bend, Indiana. “I’ve never seen cases in July, or close to July,’’ she added. She recently treated two babies who were infected with RSV. Both needed oxygen treatment to help them breathe. The U.S. Centers for Disease Control and Prevention released a health advisory on June 10.