But now they're dying and what's killing them is a mystery, reports CBS News Correspondent Bobbi Harley.
"These are very tough animals, and it's just frightful to see something that can bring them low in a matter of weeks, " says Dr. Brian Bowen, a Wildlife Geneticist at the University of Florida.
It began in late October when boaters started spotting turtles that appeared to be dead, too weak to move or even blink.
Of the 12 brought to a specialized hospital in the Florida Keys, six have died and three others are critical.
"It's frustrating, they're not getting better. I mean, usually we can save 75 to 80 percent of the turtles that come in and for us to lose half of these is very, very frustrating," said Richie Moretti of the Turtle Hospital.
Scientists' best guess is a previously unknown strain of herpes. The disease has shown up before, but never with such deadly ferocity.
A big problem is that no one knows exactly how many sick turtles are out there. One indication may be the number of stranded turtles found in the last three months. It's more than double the same period last year.
Officer Steve Jahn, who monitors wildlife along the Florida Keys for the Florida Fish and Wildlife Commission, says he's taken a lot more calls about sick turtles. He notes, "In the month of November, we had 43 complaints and the whole year we've only had 315."
The race is on to find answers in time to head off an epidemic. In four months the population will swell as loggerheads by the thousands return to Florida beaches to lay their eggs. If the disease spreads unchecked, it could devastate the species.
Almost extinct everywhere else, Florida's one of the last strongholds for this ancient animal.
Veterinarian Tim Tristan explains, "You gotta keep working. You can't quit on these guys, they're not quitting on themselves, they're not quitting on us, we're not going to quit on them."
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