CHICAGO — Three of the top U.S. medical organizations issued an open letter Monday urging Americans to wear masks, social distance and wash hands often to help stop “the worst public health crisis in generations.”

In the absence of a federal mandate, the American Medical Association, American Nurses Association and American Hospital Association issued the plea and said steps taken early on that helped slow the spread of COVID-19 “were too quickly abandoned.”

“We are not powerless in this public health crisis, and we can defeat it in the same way we defeated previous threats to public health — by allowing science and evidence to shape our decisions and inform our actions,” the letter said.