Thirty-one Chinese tourists returning from Thailand recently tested positive for norovirus, a common cause of acute gastrointestinal illness, in Hefei, capital of Anhui province.

Effective measures had been taken immediately after the tourists returned to China on March 28 to prevent the virus from spreading, according to the Anhui Entry-Exit Inspection and Quarantine Bureau.

It wasn't until Monday morning that the bureau released information about the case during a news conference.

The tourists took flight E3817 from Bangkok to Hefei. After arriving at the airport, some of the passengers reported to quarantine workers at the Hefei airport that a group of them had been suffering acute gastrointestinal illness since March 26.

"They ate some seafood, including shark fin, together in Bangkok and Pattaya on March 26," said Bao Liming, deputy director of the Anhui inspection and quarantine bureau.

The illness had not been cured, though the tourist guide had given them some pills, Bao said.

Disinfection measures were taken at the airport and on the airplane, and the tourists were sent to a hospital for treatment immediately, according to the bureau's press release.

An emergency response was activated by the bureau March 28 and had been canceled by Monday based on test results, Bao said, adding all of the infected tourists have recovered from the illness and no more cases have been found in subsequent days.

Norovirus is transmitted mainly through contaminated food and water, Bao said, citing media reports saying multiple recent flights from Thailand to China have had virus carriers.

Yang Xiao and Tao Kun contributed to this story