New Hampshire health officials say 18 people were sickened with a gastrointestinal illness after attending a private function at a restaurant co-owned by a Democratic congressman that's popular with politicians.

The state Department of Health and Human Services said Friday that people who were sickened with norovirus attended a private function Nov. 24 at the Puritan Backroom in Manchester.

The state said one person who attended the event later died but could not say whether the death was related to the outbreak.

"We do not know if this person’s death is related to this outbreak," Jake Leon, a spokesman for the state Department of Health and Human Services, told the Union Leader newspaper. "Because norovirus is so transmissible, it can be difficult to determine what may have been the source of these illnesses."

The restaurant is co-owned by New Hampshire Congressman Chris Pappas and has hosted events featuring politicians over the years, including future presidents like George H. W. Bush and Bill Clinton. It celebrated its 100th anniversary in April of 2017.

In a statement to the Union Leader, Puritan Backroom co-owner Eric Zink said the restaurant doesn't know how the norovirus was spread. He said the state has not been able to pin the outbreak o a specific food item.

Norovirus can cause what's sometimes referred to "the stomach flu" but is not related to influenza. It's highly contagious and can be spread through contaminated surfaces and ingestion of contaminated food or water.

The Associated Press contributed to this report.