SINGAPORE (AP) — The United States team at the swimming world championships in Singapore is battling a case of “acute gastroenteritis” that compromised performances on Sunday, the opening day of eight days of competition in the pool.

Nikki Warner, the spokeswoman for USA Swimming, confirmed the outbreak to The Associated Press and said it had its roots at a training camp the American team held in Phuket, Thailand, before arriving 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.

Defending Olympic 100-meter butterfly champion Huske was withdrawn from preliminary heats of her event on Sunday. The team initially said she was pulled to focus 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.