The contamination arrived after a sewage spill in Maryland seeped into Northern Virginia due to miscommunication.
Twenty-seven people in all became ill, according to the Loudoun County health department.
The Maryland Department of Environment says it was made aware Nov. 13 of a report regarding the illnesses linked to the oysters.
The department says it took steps to put an emergency shellfish harvesting closure in place effective that day.
Officials say the sewage overflow occurred in late October at a Metcom pump station.
The Maryland Department of Environment reported on the incident, however the information was reportedly not conveyed to the shellfish program, which would have closed the nearby harvesting area.
Maryland state leaders say they’re now working on improving their coordination within programs through retraining and more.
The water conditions in the affected area were reportedly investigated and the emergency closure lifted Nov. 20 when the data showed the oysters can be harvested and public health protected, according to officials in Maryland.