PHOENIX — Tests show a wildlife biologist at Grand Canyon National Park most likely died of plague.

The death of 37-year-old Eric York last week triggered a health scare that led to about 30 people being given antibiotics as a precaution.

An official who has been briefed on the test results confirmed Friday that they were positive for plague, although the test isn't absolutely conclusive. The official didn't want to be named because an official announcement hasn't yet been made.

The National Park Service plans a midday press briefing to formally announce the test results.

York became ill on Oct. 30 and called in sick for a couple of days before being found dead in his home on Nov. 2. As a wildlife biologist, he often came into contact with wild animals that can carry the plague or in areas where rodents congregate. He also was an avid outdoorsman and liked to hike and hunt.

Park Service officials said they suspected the plague or hantavirus, another sometimes-fatal disease endemic to the Southwest.

Health officials in Arizona warned in September that the plague appeared to be on the rise and that more cases were likely after an Apache County woman was infected with the disease.

That case, the first human infection reported in Arizona since 2000, followed the discovery of an outbreak of the disease in prairie dogs in Flagstaff in August.

Arizona health officials have been wary about a plague outbreak because of greater activity in New Mexico and other nearby states in the past year. In New Mexico, the plague caused the death of a 3-year-old boy in June.

An average of 13 plague cases are reported in the United States each year. Fourteen percent of cases are fatal, according to the Centers for Disease Control and Prevention reports.

While Arizona health officials say the disease appears to be on the rise in the state, Centers for Disease Control and Prevention spokeswoman Lola Russell said plague cases weren't increasing this year on a national level.

Plague is transmitted primarily by fleas and direct contact with infected animals. When the disease causes pneumonia, it can be transmitted from an infected person to a non-infected person by airborne cough droplets.

People were being warning to avoid contact with animal burrows and stay away from dead animals.

Cases are treatable with antibiotics, but the Centers for Disease Control and Prevention reports that up to 50 percent are fatal if the disease causes pneumonia.

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Associated Press reporter Jacques Billeaud contributed to this report.