While testing is falling well short of demand across the country, public health officials say the unique risk in Florida, where 27 percent of the population is over the age of 60, and eight people have already died, creates a serious need for better testing…

Mr. DeSantis, a Republican, was at first reluctant to mandate mass cancellations of public events or restrictions on movement. But on Tuesday, after photos on social media showed crowds of tanned young people sunning shoulder-to-shoulder on the beaches, the governor shut down bars and nightclubs for a month and ordered restaurants to cut their seating capacity by half. He refused to close beaches, though groups will have to be no larger than 10.

The crowds of young people on the beaches have been one of the biggest sources of concern.

“These kids are asymptomatic,” Dr. Boros said. “They are not going to cancel spring break. They are going to come down here, drink beer and have fun. We have pretty much done ourselves in.”