She gave an interview to The Daily Show in 2014, claiming vaccines are “full of toxins.” The title of the segment was “An Outbreak of Liberal Idiocy” and compared the progressive anti-vaccination movement to conservative climate-change denialists.

“You can line up the doctors from here to down the block refuting me, but I’m not going to change my mind,” Ms. Pope said.

As Ms. Manookian often notes in her biographical information, she had a career working on Wall Street in the 1990s and early 2000s. But then, when she was 28, according to her site, she got a “ton of travel vaccines,” which led to a “ton of health problems.”

The right judge

On July 12, 2021, when Ms. Pope and Ms. Daza filed their lawsuit, the Tampa division randomly assigned it to its newest judge, Judge Mizelle, a conservative jurist appointed by President Donald J. Trump in November 2020. It was a boon for the plaintiffs.

“They got lucky with a judge that was sympathetic to their ideology,” said Lawrence O. Gostin, a Georgetown University professor of global health law.

Once their team had the winning ticket, they fought to keep it. On Oct. 15, lawyers representing the C.D.C. and the White House pushed to transfer the case to a different judge in the same district, Paul G. Byron, to “avoid the probability of inefficiency.” Judge Byron, who was appointed by President Barack Obama in 2014, was already handling a similar case against the C.D.C. involving a man who said his anxiety made it impossible for him to wear a mask, preventing him from flying. The plaintiffs argued that the cases were quite different and Judge Mizelle denied the motion to transfer.

On April 18, the day the mask mandate had been scheduled to expire — five days earlier, the C.D.C. had extended it by two weeks — Judge Mizelle issued her ruling. She focused, in part, on the Public Health Service Act, a law created in 1944 that gives federal officials the authority to make and enforce regulations to prevent the introduction of a communicable disease from foreign countries and its spread between states. Those regulations could include “inspection, fumigation, disinfection, sanitation, pest extermination, destruction of animals,” the law states, “and other measures” that the authorities judge “may be necessary.”