One day last October, Eric York lugged the carcass of an adult mountain lion from his truck and laid it carefully on a tarp on the floor of his garage.

The female mountain lion had a bloody nose, but her hide bore no other signs of trauma. York, a biologist at Grand Canyon National Park in Arizona, found the big cat lying motionless near the canyon's South Rim. He was determined to learn why she died.

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Because the park lacks a forensics lab, he did the postmortem in his garage, in a village of about 2,000 park employees.

Epidemic experts can only speculate about what happened next. When York cut into the lion, he must have released a cloud of bacteria and breathed in. On Nov. 2, York was found dead, a 21st-century victim of plague, the disease that in the Middle Ages turned Europe into a vast mortuary. He was 37.

The case mirrors events that have promoted a global surge in epidemics, among them influenza, HIV, West Nile virus and SARS. A study released this year in the journal Nature reported that about 60% of epidemics begin when a microbe makes the leap from an animal into a human.

"What will be the next emerging disease? The one we least expect," says David Morens of the National Institute of Allergy and Infectious Diseases.

Word of York's death flew among those who worked at the famed natural attraction, which draws 5 million visitors a year. For public health experts, it provoked concerns that plague might make a comeback. Experts from the National Park Service, the Centers for Disease Control and Prevention and the Arizona Department of Health converged on the park.

Fortunately, their investigation found only 49 people who had come in contact with York. All were treated with antibiotics. None became ill, says David Wong, a National Park Service epidemiologist.

"We identified his contacts even before the autopsy results were in," Wong says. "Within minutes, we were calling folks to tell them to come in. We opened the clinic on a Sunday."

The investigators who combed occupied areas of the park also were relieved to find no evidence of the rodent die-offs that prompt plague-infected fleas to leap to people and feast on them instead of the animals, Wong says. Massive flea migrations, prompted by widespread rodent deaths, caused Black Death in Europe during the Middle Ages.

Both York and the mountain lion had pneumonic plague, a lung infection that spreads through a cough or a sneeze.

"Pneumonic plague is a highly fatal disease," Wong says. "The death rate can be as high as 50% even with treatment."

Concerned about big cats

York was widely known for trapping and collaring big cats to study their movements and protect them from encroaching humans, says Charles Higgens, director of public health for the National Park Service.

York's friends say he could make a mountain lion trap with toothpicks, says Launie York, the biologist's mother. She says her son loved the woods around the family farm and was forever storing specimens in the family freezer.

"We had a saying here: 'If it's in a black plastic bag, don't open it. It isn't dinner,' " she says.

Before his fatal encounter with the mountain lion, York got to know the big cat well. During his two years at the park, York tracked, trapped and collared her. When she gave birth to three kittens, he ear-tagged them so that he could identify them when they were old enough for their own telemetry collars.

Then, on Oct. 25, the lion's collar sent out a mortality signal, indicating that she hadn't moved in 24 hours. When York located her carcass, her kittens were nowhere to be found. His notes suggest that he believed she may have been killed in a fight with a male, because of blood pooled around her nose. But York wasn't satisfied with guesswork, so he decided to do an autopsy at his home.

Ambushed by germs

Although plague is endemic west of the Mississippi — brought here in the 1800s by flea-infested rats on ships ferrying Chinese railroad workers to the USA — York had little reason to suspect it. Mountain lions usually stalk bigger game than rodents. But this lion had kittens that had to learn to hunt.

When York became ill, he visited the park's clinic, Wong says. On Oct. 30, clinic staff diagnosed a flu-like illness and sent him home. It was there, three days later, that a roommate found him lying motionless on the couch.

Wong says York's toughness and self-sufficiency may have cost him his life. "He was a tough guy. He gutted out more than you or I or almost anyone else would."

He says the case has prompted the National Park Service to begin working with colleagues at the CDC and at state and local health departments to identify diseases within the park system that might pose a risk to the 276 million people who visit every year, as well as the many people who might be exposed once park visitors return home.