One of the main drinking water reservoirs in Baltimore is contaminated with a microscopic parasite that can cause gastrointestinal issues, however, the city says risk of infection is low.

City officials announced Thursday that Druid Lake Reservoir tested positive for small amounts of cryptosporidium, a microorganism commonly found in lakes and rivers.

“The risk has been determined to be low,” Tamera Green, Baltimore’s chief medical officer, said during a Thursday press conference. “We are working with the state and the Department of Public Works and will continue to monitor the situation.”

The reservoir provides drinking water to residents in about half of the city, Baltimore County and Howard County.

The city says most residents will not need to take any action, however, people who are immunocompromised, children and elderly people should drink water that’s been boiled or bottled water until further notice.

An interactive map is available online to look up where in the city and surrounding counties the water may be contaminated.

DPW regularly test for cryptosporidium in the drinking water each month.