Tibetan nomads who once eked out a living as farmers or yak-herders have found a lucrative new business in recent years — caterpillar fungus.

Also known as cordyceps, the fungus grows on caterpillars, killing and mummifying them underground before growing a stalk that can be picked like a mushroom in spring. The fungus is relatively rare, but flourishes in the high altitudes and low temperatures of the Tibetan Plateau.

The fungus has been popular for centuries as a traditional cure for ailments ranging from asthma to impotence. But demand has skyrocketed as China's middle class expands, says Getty photojournalist Kevin Frayer.

"The insatiable demand in the last decade for use in Chinese medicine has driven the prices to a point where good quality fungus can be worth more than their weight in gold," Frayer told The WorldPost in an email.

Frayer, who is documenting the lives of nomadic Tibetan communities for an ongoing project, said everyone he met kept talking about the annual harvest, so he went along last month.

"The mountains of Tibet are vast and many of the places the cordyceps fungus are harvested from are quite remote … so you need to trek in and climb to access the areas," he said. "The landscape is incredible and exceptionally beautiful but the terrain is also hard and unforgiving."

The harvest usually lasts one month in May and June, and has become a major part of the region's economy.

Many Tibetan nomads have given up on traditional trades in favor of the annual caterpillar fungus "gold rush," Frayer explained.

"For millennia they have been yak and livestock herders living off the land, but now more and more people are relying on the fungus to pay their bills," he said. "The harvest gives these communities [a] chance to make in weeks what normally might take a year or more."

Kevin Frayer/Getty Images

Cordycep fungus sellers in the town of Sershul on the Tibetan Plateau on May 20, 2016. The fungus has become a major part of the local economy, but harvesters fear both demand and supply are dwindling.

The trouble is, it's getting harder and harder to hunt down the caterpillar fungus, which can't grow fast enough to keep up with Chinese appetites. Tibetan nomads told Frayer that the yield from this year's harvest was the lowest they'd ever seen. Environmentalists have also warned about the long-term impact of the harvest on the sensitive environment of the Tibetan Plateau.

Meanwhile, prices for the fungus are falling, and harvesters fear China's crackdown on corruption could hurt demand for the product as a high-value gift for officials. A recent health warning about arsenic levels in caterpillar fungus products is a further headache for cordyceps hunters.

"The locals know it's a false economy, or at least temporary in many ways — one Tibetan man referred to the fungus as "fool's gold" and he worried that one day they will be worthless," Frayer said.

One day when he was documenting the harvest, Frayer said, the altitude started to get to him and he laid down for a nap. "As I dozed off, I felt a nudge from a man picking cordyceps fungus nearby. He told me it is bad luck to fall asleep in the mountains," he recalled. "'It might anger the mountain gods,' he told me, 'and that would be bad for all of us.'"

See more of Kevin Frayer's photos from the cordyceps harvest below.