SALEM — The number of coronavirus cases continues to rise at the Oregon State Penitentiary, which this week became the source of the single largest known outbreak of the virus in the state.

On Friday, the Oregon Department of Corrections said a total of 141 people at the prison — 115 inmates and 26 employees — have tested positive. A southeast Portland nursing home is the source of the state’s second largest outbreak.

Four inmates at the maximum-security prison in Salem have been treated for the disease at local hospitals, including one man who died Wednesday. It was the first inmate death from coronavirus in Oregon.

The man had been transferred May 18 to Salem Hospital and was tested that day. May 19, the results confirmed he had the virus. He died the next day.

Like the vast majority of people who have died from the illness in Oregon, the man had an underlying health condition, prison officials said. Citing the confidentiality, the state declined to identify the man, saying only that he was between 50 and 60 years old.

Dr. Christopher DiGiulio, the chief of medicine for the Corrections Department, said inmates’ movements within the prison are restricted to contain the spread of the illness.

Built in 1866, the state penitentiary, a hulking complex of imposing cellblocks, was not designed with a pandemic in mind, DiGiulio said.

“That is one of the most difficult aspects and the biggest challenge managing this,” he said. “The building is over 100 years old. It was not designed for social distancing. It was probably designed before germ theory was established. So the idea of social distancing inside of OSP is difficult.”

He said hundreds of men live “in very close quarters on multiple tiers.”

Corrections officials had been moving almost all infected inmates to Coffee Creek Correctional Institution in Wilsonville, where the infirmary is equipped with rooms that limit air circulation. But the rising number of cases has prompted the agency to shift away from that practice, DiGiulio said.

The infirmary continues to take people who are considered medically vulnerable and have a serious case of the virus, but people with symptoms who are in better shape are treated in a converted housing unit at Coffee Creek, the state women’s prison.

DiGiulio said people who test positive but don’t have any symptoms and are otherwise healthy have remained in quarantine at the state penitentiary.

Inmates are routinely screened for symptoms, he said, but testing remains a hard sell among the men who fear they will test positive and end up moved away from their cells and placed in an even more restrictive setting.

Only about 15 percent of nearly 2,000 inmates at the penitentiary have been tested. Statewide, about 4 percent of inmates have been tested in a population of about 14,000. The state does not have data on numbers of workers who have been tested since those are done by employees’ medical providers.

“We have the capacity to test pretty much as many patients as we would like but there is some natural resistance to (inmates) wanting to be tested,” he said.

The state this week also began antibody testing in its prisons, though no results are in yet. The tests, which detect the presence of coronavirus antibodies, were offered at Shutter Creek Correctional Institution, which had 25 cases of the disease. The agency said the testing “will help us quantify the breadth and scope of the virus” in prisons where the disease is present.

It is unclear how much long-term protection antibodies offer, DiGiulio said.

“We don’t know for sure if these antibodies are completely protective,” he said. “We don’t know if they are partially protective. At this point, there is still quite a bit of uncertainty.”