Yosemite National Park is one of the most visited places in the U.S. with more than 3.7 million people treking to the California hotspot each year. Some of those nature lovers may be at risk for a serious disease, however.

It was recently reported that approximately 1,700 Yosemite visitors who stayed in tent cabins this summer may have been exposed to a deadly rodent-borne virus — a disease that has already claimed the lives of two people.

After learning that a Pennsylvania visitor’s death was caused by hantavirus, Yosemite officials sent emails Monday evening to those who stayed in the “signature tent cabins” in Curry Village between mid-June and late August, said park spokesman Scott Gediman. Letters were sent to visitors whose email addresses were not on record.

The fatality marked the third confirmed case of the rare rodent-borne disease linked to the park. Last week, park officials said a 37-year-old Bay Area man had died and an Inland Empire woman in her 40s was recovering after being exposed to the virus. Park officials believe there may be a fourth case but had yet to receive confirmation Tuesday.

All four stayed separately at the signature tent cabins in June, Gediman said. Officials have traced the outbreak to deer mouse droppings in the area.

Jana McCabe, a Yosemite park ranger, called the hantavirus outbreak “unprecedented.” Though the park dealt with the same type of outbreak in the past (2000 and 2010), neither instance caused a fatality, and since then employees of the park have been trained on proper hantavirus protocol.

Since this most recent outbreak, the park has stepped up its response, implementing “rolling closures” of the cabins for deep cleaning, McCabe said. Crews are tearing down interior walls to look inside and repairing holes where mice could get into the structures. Meanwhile, Yosemite is suffering a reputation setback. As word continues to spread about the deadly hentavirus outbreak, the park will undoubtedly see a drop in tourist attendance.

There have been only 587 documented cases on hentavirus in the U.S. since the virus was indentified in 1993.