Before Mullet Key was a fort, and alongside the fort once it came, the island had another federal job: it was a gate against disease. For nearly half a century a quarantine station stood here, where the ships coming in from the Gulf and the Caribbean to the growing port of Tampa were stopped, inspected, and, if necessary, held, so that the epidemics that haunted the age, yellow fever above all, would not ride into the city on an incoming hull. The island guarded the bay against contagion as much as against any fleet.
In the late nineteenth century, the port cities of the Gulf and the South lived in real fear of epidemic disease. Yellow fever, cholera, and smallpox could arrive by sea and devastate a city in a season, and Tampa, booming on phosphate, cigars, and railroads, and trading heavily with Cuba, was acutely exposed. The federal answer was the maritime quarantine: a station placed at the entrance to a harbor where every arriving vessel could be examined before it was allowed to proceed to the docks. Mullet Key, sitting astride the only deep channel into Tampa Bay, was the natural place for Tampa's.
The Mullet Key Quarantine Station was established around 1889 and run by the U.S. Marine Hospital Service, the agency that would grow into the U.S. Public Health Service. Ships inbound for Tampa were required to stop at the station, where medical officers boarded to inspect the vessel, its cargo, its crew, and its passengers for signs of contagious disease. A ship from an infected port, or one with sickness aboard, could be detained, its people held in isolation, its holds and goods fumigated and disinfected, until the danger had passed. The station had the buildings such work required, quarters, a wharf, facilities for detention and disinfection, a small federal medical outpost on a barrier island.
The quarantine station predates Fort De Soto by nearly a decade and ran right through the fort's active life and beyond it. For years the island carried two distinct federal missions at once: the Army's coast-defense post at the southwest point, and the Public Health Service's quarantine station, the soldiers and the medical officers sharing the same mosquito-ridden key. The station's screening role gradually wound down in the early twentieth century as yellow fever was conquered, mosquito control advanced, and quarantine practice changed, and it closed around 1937, about the time the Army was selling the island to the county.
The quarantine station is easy to forget beside the drama of the guns, but it tells an essential part of the island's meaning. Mullet Key was always, by its geography, a threshold, the last island a ship passed before the bay and the city. The Army used that geography to guard against enemies; the Public Health Service used the same geography to guard against disease. Both were betting on the same fact: that whoever holds the mouth of the bay controls what reaches Tampa. For half a century, what the island held back was not a fleet but the fever.