A team from the Centers for Disease Control and Prevention has arrived in Sioux Falls, South Dakota, to help with the Smithfield Foods coronavirus hot spot, which has become the biggest single source of cases in the United States.

Smithfield also closed a plant in Martin City, Missouri, and another in Cudahy, Wisconsin. Smithfield said a small number of employees at the Wisconsin and Missouri plants have tested positive for COVID-19.

Eighty of South Dakota's 180 new COVID-19 cases are employees of the meat-processing company, bringing the total to 518 Smithfield Foods employees who have tested positive. There are also now 126 total cases of non-employees that became infected when they came into contact with a Smithfield employee, according to the South Dakota Department of Health.

The 518 employees and 126 non-employees connected to Smithfield makes it the largest cluster in the country (644), according to tracking by The New York Times. The previous top cluster was 585 cases aboard the USS Theodore Roosevelt in Guam.

Smithfield announced Sunday that it would be closing its Sioux Falls plant indefinitely Wednesday. The plant has 3,700 employees.

Gov. Kristi Noem said the state is "aggressively testing" Smithfield employees and people who have come into contact with them, as well as getting people into isolation as soon as possible. Noem has not yet issued a statewide stay-at-home order for South Dakota, one of seven states that have not issued orders (along with Arkansas, Iowa, Nebraska, North Dakota, Wyoming and Utah).

At the Martin City plant, six to nine people, including managers, contracted COVID-19, Martin Rosas, president of United Food & Commercial Workers Union Local 2, told The Associated Press.

One of the deciding factors in closing the facility was because the shuttered Sioux Falls plant supplies the Missouri operation with hams. The Martin City facility produces spiral hams.

The union was told that Smithfield plans to be back in full operation in Martin City by April 30, Rosas said. Employees will continue to receive their full 40-hour pay in accordance with their collective bargaining agreement.

The CDC team is scheduled to tour Smithfield’s South Dakota plant on Thursday morning and create a checklist of items to complete before the plant can reopen, Noem said during a press conference on Wednesday. Noem said she's working with federal officials and Smithfield leaders to get the plant back online to provide relief for pork producers and the food chain.

Health Secretary Kim Malsam-Rysdon said they requested the CDC bring in occupational health experts to help the state understand Smithfield's coronavirus situation and what the company can do to protect its workers. Smithfield is cooperating with the state, and they look forward to the mitigation efforts so the plant can reopen, Malsam-Rysdon said.

The Associated Press contributed to this report.