As of Tuesday, at least 222 recruits at Lackland Air Force Base, part of Joint Base San Antonio, had been diagnosed with the flu and four had been hospitalized, the two people familiar with the matter told ABC News.

This marks a sharp increase from the 159 cases and two hospitalizations reported last week. The death of one recruit remains under investigation, though it is not yet clear whether it is tied to the outbreak, the sources said.

The outbreak is unfolding just two months after Defense Secretary Pete Hegseth made the annual flu shot optional for troops, scrapping the military's requirement for it, which dates to 1945, a move breaking with longstanding public health directives.