Two weeks ago it was announced that an outbreak of a rare disease had been linked to a campsite at Yosemite National Park. At the time, one man had died and a woman was gravely ill from hantavirus pulmonary syndrome, a deadly virus spread by rodents. The park suggested that people seek out medical treatment if they developed any symptoms, which are generally flu-like at the outset.

Since that time, it has become clear that the hantavirus outbreak isn’t yet contained. The infected woman succumbed to the disease and four new cases were identified. Just before the holiday weekend, it was made clear that the national park was working with the California Department of Public Health (CDPH) and the U.S. Centers for Disease Control and Prevention (CDC) to investigate the outbreak and identify new cases.

“CDPH is working closely with the National Park Service and the Centers for Disease Control and Prevention to further investigate the cluster of Hantavirus Pulmonary Syndrome cases in Yosemite and reduce the risk of other visitors becoming ill from this virus,” said CDPH Director Dr. Ron Chapman on August 30. “CDPH is continuing to monitor cases of Hantavirus Pulmonary Syndrome in persons who visited Yosemite National Park.”

The park has begun scaling up its public awareness campaign, and has begun contacting campers who have stayed at Yosemite’s Curry Village in recent months. Around 3,000 people have been contacted and informed of hantavirus symptoms. The campsite has not been closed.

“The park and public health officials are contacting visitors and raising awareness in the medical community to increase the chances that any additional cases that may be incubating will be successfully diagnosed and treated early,” stated Dr. Danielle Buttke, an epidemiologist with the National Park Service Office of Public Health.

Luckily, the four people confirmed to have hantavirus who haven’t died are reported to be improving or recovered. Still, hantavirus can be extremely deadly. While the early symptoms of the disease are generally flu-like, they can include coughing, malaise, headache, nausea, vomiting, and shortness of breath, according to the U.S. National Institutes of Health. If not treated promptly, the virus can cause lung, kidney, and/or heart failure. The disease is spread by rodent urine, droppings, and saliva, particularly of deer mice.