D.C. Fire and EMS officials say the most recent firefighter to be confirmed is recovering at home. Her last day on duty was March 9.
Health officials are trying to determine where the firefighter became infected.
She was stationed at Engine Company 20.
According to FOX 5's Paul Wagner, the number of firefighters quarantined could go higher.
After the second confirmed case, new rules were put in place ordering firefighters to be checked for illness before their shifts.