Health authorities are desperately trying to prevent every public pool patron's worst nightmare by urging anybody with diarrhoea to stay away.
There have been 149 cases of the parasitic intestinal infection cryptosporidiosis across NSW so far this December, with more than half occurring in kids under 10 and overall numbers on the rise.
"Anyone with the symptoms should help protect other swimmers by staying out of pools for at least two weeks after the diarrhoea has stopped," Health Protection NSW Director Dr Jeremy McAnulty said on Thursday.
Parents are being urged to take children on regular bathroom breaks and ensure kids who aren't toilet-trained wear waterproof, tight-fitting pants over their swimmers.
Nappies should be changed in a bathroom - not at the poolside - and children should be washed thoroughly before going swimming, authorities say.
