In California's Sierra Nevada mountains, yellow-legged frogs survive at some of the highest elevations of any amphibians in North America.

As with all frogs, their skin is essential.

On a hot day, they drink water through it.

And they absorb the sodium and potassium their hearts need to work.

When they're at the bottom of a lake -- which is where they spend the winter -- they breathe through their skin.

And those black dots ooze mucus that protects the skin from infection.

But it doesn't always work.

In the late 1970s, a fungus started getting into the frogs through their skin, moving through the water, or passed on by other frogs.

Those tiny transparent beads you see moving under the microscope are spores of chytrid fungus.

The spores burrow down into the frog's skin.

The skin gets irritated.

Then it sloughs off.

It can no longer absorb sodium and potassium.

Eventually the frog dies.

Chytrid fungus has been like the "black death" for amphibians.

It has decimated some 200 species around the world.

In the Sierra, chytrid fungus swept across the mountains and in three decades killed almost every single yellow-legged frog.

So scientists did an experiment to see if they could save the species.

They grew some frogs from eggs.

Then they infected them with chytrid fungus.

This was risky.

But they hoped to give the frogs something like a vaccine.

The original experiment took place at the University of California, Santa Barbara.

Now it's being replicated at the San Francisco Zoo.

The frogs got sick.

Their skin came off.

But then researchers treated the frogs with a liquid antifungal that stopped the chytrid before it could kill the frogs.

When the frogs were nice and healthy again, researchers re-infected them with chytrid fungus.

To their surprise, the Santa Barbara scientists found that all the frogs they had immunized survived.

Every one of them.

Now the San Francisco Zoo has immunized 124 mountain yellow-legged frogs.

And they're returning them to their mountain lakes.

[WOMAN: "There you go!"]

The idea is that next time chytrid fungus ravages the lakes, as it inevitably will, these frogs will have immunity.

Some will survive.

They'll get a chance to reproduce, and maybe, slowly, a more robust population of frogs will once again thrive here in the alpine lakes of the sierras.