PORTLAND, Ore. (KOIN) — Providence Portland Medical Center had a COVID outbreak last month among patients who were not originally sick with the virus, spreading it to workers too.

13 patients were sickened on an 18-bed floor. All of them were asymptomatic or had mild symptoms. Of the 217 workers on the floor, 36 tested positive. 24 have since returned to work.

The unit was closed for a deep cleaning and has not yet reopened. Providence is still investigating the outbreak and does not have a known cause. They are also conducting genomic testing.

The last positive test connected to the outbreak was on Dec. 31.