MISSOULA, Mont. (AP) — The Montana State Prison has locked down due to an outbreak of the coronavirus in the facility, the state Department of Corrections said.

The lockdown starting Friday occurred after the number of confirmed cases of the coronavirus in the prison jumped from eight to 36, the Missoulian reported.

The Department of Corrections said 23 prison employees have also tested positive for the coronavirus.

“To help keep the virus from spreading at MSP, the facility has been placed on quarantine, allowing extremely limited movement of offenders within the facility,” Department of Corrections spokeswoman Carolynn Bright said. “At this time, MSP inmates who are positive for COVID-19 are either being isolated in their cells or cohorted with other COVID-19-positive inmates."

The prison in southwest Montana houses about 1,500 inmates and employs about 700 staff members.

The number of infections is thought to be far higher because many people have not been tested, and studies suggest people can be infected with the virus without feeling sick.

For most people, the new coronavirus causes mild or moderate symptoms, such as fever and cough that clear up in two to three weeks. For some — especially older adults and people with existing health problems — it can cause more severe illness, including pneumonia, and death.