(Newser) – Yosemite National Park is battling a suspected norovirus outbreak that has caused nearly 200 people to fall ill. Some 170 visitors and employees have experienced a gastrointestinal illness, with the "overwhelming majority" of cases consistent with norovirus—a highly contagious virus marked by stomach pain, nausea, and diarrhea. It's been confirmed in two reported cases, according to a Thursday statement.

"When you are sick with norovirus, you can shed billions of virus particles in your vomit and poop. It only takes a few of these particles to make someone sick," the CDC notes, per CBS News. Most suspected cases occurred in people who visited Yosemite Valley around the first week of January. The park reports a "significant decline" in illnesses in recent days though it remains under "enhanced sanitation protocols." (Read more Yosemite National Park stories.)