Labor Day weekend is a popular time to head outdoors with the family, fire up the barbecue, and do some camping. Unfortunately, rangers at the Curry Village campground in Yosemite National Park—a popular destination for families—are warning visitors to the site after two people died from a dangerous disease.

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Four people who stayed at the campground in June contracted hantavirus pulmonary syndrome—a potentially-fatal virus spread by the feces and urine of rodents. “Of the 587 documented U.S. cases since the virus was identified in 1993,”>/a> reports, “about one-third proved fatal.”