SINGAPORE (AP) — The U.S. contingent at the swimming world championships in Singapore is grappling with an “acute gastroenteritis” outbreak that affected their performance on Sunday’s opening day of the eight-day swimming event.

Nikki Warner, USA Swimming’s spokeswoman, confirmed to The Associated Press that the outbreak originated during a training camp in Phuket, Thailand, prior to their arrival in Singapore.

She said all American swimmers had traveled to Singapore. She declined to say how many had been affected with the infectious diarrhea.

At least three clearly have been impacted: Torri Huske, Luca Mijatovic and Claire Weinstein.

The defending Olympic 100-meter butterfly champion Huske was withdrawn from her event’s preliminary heats on Sunday. The team initially mentioned she was withdrawn to concentrate on relay events.

American 16-year-old Mijatovic swam the prelims of the 400 freestyle. His time of 3:59.68 was almost 15 seconds off his entry time, and it was clear he was under duress as he swam.

Weinstein, a promising 18 year old, was also pulled from the 400 freestyle.

Katie Ledecky, the most famous swimmer on the American team, performed normally in her preliminary session on Sunday and did not appear to be affected.

The United States swim team is trying to rebound from a difficult time a year ago at the Paris Olympics. The Americans won only eight gold medals, their lowest total since the 1988 Seoul Olympics.

That relatively low total still topped the medal table followed by Australia.

The men won only one gold medal in an individual event in Paris, and that group is hoping to rebound in Singapore with a very young men’s team. Head coach Greg Meehan said it’s the youngest in memory.