The firefighters were moved to isolation at their homes.
All employees at the city's three fire stations are now undergoing testing, and the department is working with Lee County Health Department to conduct the testing and contact tracing.
All fire stations have since been cleaned and sanitized. The stations will all be sanitized again at shift change.
"One of my highest priorities is the safety and health of all Fire Department employees," Fire Chief Wayne Barber said. "The Sanford Fire Department follows and exceeds, where possible, the Centers for Disease Control and Prevention guidelines while providing emergency services to our community."
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Subsequent quarantines for employees exposed to the infected firefighters is causing a staff reduction in the department, but Sanford officials do not expect that to cause a noticeable disturbance in service.