It wasn’t until much later that researchers discovered that chronic wasting disease belongs to a small group of conditions caused by prions. But other prion diseases are known only to affect livestock or people, not wildlife.

Scrapie, for example, is a deadly disease that afflicts sheep. A number of studies indicated that bone meal contaminated with scrapie prions passed the prions to cows. The cows developed a prion disease of their own, called bovine spongiform encephalopathy, nicknamed mad cow disease.

In rare cases, people who ate beef from the sick cows developed prions in their own brains. As of 2016, 231 people had died from the condition worldwide.

Scientists long suspected that deer and related species developed chronic wasting disease by picking up scrapie from sheep flocks kept at Colorado State University. The disease then turned up in other states and Canada as animals were shipped to private game farms.

But Dr. Zabel now believes that the birth of chronic wasting disease may be more complicated.

Prions are misfolded versions of a naturally occurring molecule called cellular prion protein. Experiments carried out in Dr. Zabel’s lab suggest that cellular prion protein in deer and related species may be unusually prone to misfolding.

“We were able to generate a new prion,” Dr. Zabel said. “Maybe this is a spontaneous disease.”

That result might explain a startling finding last year: researchers came across a prion-riddled reindeer in Norway, the first time chronic wasting disease had been found in Europe. Since then, two more have been found, and Norway in April approved the culling of over 2,000 reindeer to stop the spread.

Dr. Zabel and other scientists are trying to figure out how chronic wasting disease has become so successful. One factor is how the prions spread through an animal’s body. They aren’t limited to the brain in deer, elk or moose. The prions also sweep through lymph nodes and the spleen.