More than 650 people have been treated for diarrhoea in one week in Carolina, Mpumalanga.
These figures indicate an outbreak of the condition, authorities said, because they exceed the usual number of 13 cases a week. Five people are still being treated in hospital.
The provincial joint medical operations committee is trying to establish the cause of the runny tummies, which is often related to contaminated drinking water.
Provincial health spokesman Mpho Gabashane said the outbreak was not cholera, which was caused by a different bacterium.
He said cholera could be fatal but diarrhoea, if treated soon enough, was quickly cured.
Gabashane said a similar increase in diarrhoea cases was reported in October last year after heavy rains similar to those that recently fell in the province.
"The recent heavy rains caused damage in and around Carolina and left the area without water and electricity for a few days," Gabashane said.
He advised rural communities to purify water by boiling it for at least three minutes before using it for cooking or drinking.
"Take one teaspoon of household bleach and add it to 20 litres of water that has been filtered through a clean cotton cloth," he advised.
"Mix well and wait for 30 minutes. Store in a clean container with a lid and store in a clean place.
"We have already activated our surveillance and health promotion teams in the area to monitor the situation and provide health education."