More than 100 schoolchildren and their teachers from Britain and the United States are in quarantine in Beijing after eight children were found to have swine flu.
Four British and four US children are being treated in hospital in Beijing. They are said to be in a stable condition.
China has quarantined hundreds of foreign visitors who have shown symptoms of the H1N1 virus this year.
One of the 52 British pupils affected told the BBC they had been visiting the Great Wall of China when they were called back, because they had previously shared a bus with a pupil from another school who had tested positive for the virus.
Another pupil said they were being treated "very, very well", with individual rooms. They have to wear protective face masks and are not allowed outside of the quarantined zone.
More than 600 Britons are on the trip. The BBC reports three of the four children were found to have high temperatures when they arrived in Beijing earlier in the week.
They were taken straight from the airport to a hospital where it was confirmed they had the virus. A fourth classmate fell ill later.
The American children had been in contact with the British group and four of them are also diagnosed as having the virus.