The United States team at swimming's world championships in Singapore have been hit with acute gastroenteritis, an official told AFP, with two competitors skipping their opening races on Sunday.
Torri Huske, a treble gold medallist at last year's Paris Olympics, and 18-year-old Claire Weinstein did not compete "so they could focus on other events", team official Nikki Warner said.
"Our team experienced acute gastroenteritis," Warner said.
"No one has pulled out of the competition.
"Two swimmers did not swim preliminary races this morning so that they could focus on their other events."
The US team arrived in Singapore after a training camp in Thailand.
Huske was scheduled to race in the women's 100m butterfly heats while Weinstein was down to compete in the 400m freestyle.
It was unclear how many members of the US team have been affected by the stomach bug.
Nine-time Olympic gold medallist Katie Ledecky did swim on Sunday morning and finished fastest in the women's 400m freestyle heats in a time of 4min 01.04sec.