A third of the fleet at the 49er sailing world championships off Buenos Aires have been hit with a mysterious stomach bug.
A stomach bug has many sailors at the 49er world championships off Buenos Aires battling bouts of vomiting and diarrhea.
"We don't really know why everyone is getting sick, but at least a third of the fleet has come down with stomach pains, diarrhea and vomiting," said Australia's Olympic gold medallist Nathan Outteridge.
Outteridge's crewmate Iain Jensen was among those hit by the bug.
Outteridge said the water wasn't clean and recent thunderstorms and rain had pushed filth onto the race course, which most sailors hadn't expected in Argentina.
"In Rio everyone knows it's dirty and takes precautions accordingly and looks after themselves, whereas here everyone gets told it's just muddy water, but there's a lot of filth in there as well," he said.
With one more day of racing before the fleet splits for the men's skiff 49er finals series, Outteridge and Jensen sit 17th, while Joel Turner and Lewis Brake are in 23rd.
David Gilmour and Rhys Mara follow in 27th, and Will and Sam Phillips are ranked 28th.
In the women's 49er FX, Tess Lloyd and Caitlin Elks are ranked 21st.
The world championship medal races will be held on Saturday afternoon (AEDT).