Three people who work in Yosemite National Park have experienced flu-like symptoms that prompted them to get tested for hantavirus, health officer Dr. Charles Mosher said at Tuesday’s Mariposa County Board of Supervisors meeting.

The Mariposa Gazette reported that while initial tests came up positive, a set of second, more-precise tests showed the three had not been exposed to the deadly Sin Nombre strain that causes hantavirus pulmonary syndrome, or HPS.

Yosemite employees may soon be offered free hantavirus tests that could help experts learn more about the outbreak, Park officials said.

Officials were still working out the details of the plan this week, NPS spokesperson Vickie Mates said.

“If the survey does go forward, we will notify our employees of this voluntary opportunity,” Mates said. “The results of the proposed survey, a common epidemiological tool, may make an important contribution to our knowledge about this rare virus.”

Testing would be conducted by California’s Department of Public Health.

Two of the nine recent Yosemite hantavirus cases involved individuals who did not come down with full-blown, lung-involved HPS. According to a 2007 article in the journal Emerging Infectious Diseases, close to one percent of workers who come into close contact with deer mice test positive for hantavirus— even if they never felt ill.

Speaking at Tuesday’s meeting, Dr. Mosher said news about the three tests shows that some park workers have been worried about possible exposure to the virus.

Medical and public health experts are intensively studying the Yosemite outbreak.