Milwaukee County Zoo workers are trying to help scientists slow the spread of a fungus that has killed millions of frogs and expunged 120 species.

The Chytrid fungus has moved rapidly through the frog populations in the Caribbean and Central America, killing nearly every croaker that contracts it. Scientists have been unable to stop the spread of disease even as the amphibians' deaths tip ecosystems out of balance.

At the Milwaukee County Zoo, three workers are collecting, disinfecting and housing a few frogs who have survived. They hope the frogs will someday be able to repopulate decimated areas.

Meanwhile, scientists are racing to answer basic questions about the fungus, including where it came from and how it spreads.

"It's kind of like what AIDS was in the 1980s," said Craig Berg, the zoo's reptile and aquarium curator, referring to the urgency of the research effort.

The disease suffocates frogs by thickening their skin so that oxygen can't flow in and carbon dioxide can't escape.

"When we find frogs who are about to die, they are standing on the very tips of their toes, straining to expose as much skin as possible" to breathe, Berg said.

Scientists suspect the fungus came from Africa, in part because they have a preserved South African clawed toad killed by it in 1938. That's the earliest evidence of the disease, they said.

By 2005, the fungus had killed frogs in the upper altitude forests of Mexico and Costa Rica and been spotted in Panama.

Panamanian researchers anticipated its arrival and began building a conservation centre where frogs could be kept and bred to later repopulate affected areas.

But the disease spread faster than anticipated, and frog experts and volunteers had to scramble to save the few frogs they could, sending them to temporary safeholds until the centre is finished next year.

"This really is a stopgap measure or an emergency response measure," said Peter Rieger, a curator at the Houston Zoo. "We are getting them out of harm's way."

Berg and two other workers at the Milwaukee zoo have focused on saving the Grenada frog. They've collected eight, and a female in their care laid eggs this year. They didn't hatch, but it was still a milestone — the first time a Grenada frog produced eggs in captivity.