Calside Primary, which has a school roll of about 170, shut on Thursday.
Dumfries and Galloway Council said pupils and staff had been struck down by an illness with "symptoms similar to gastroenteritis".
Public Health Consultant Dr Nigel Calvert said such bugs could be highly infectious.
He said anyone with symptoms should stay away from school or their workplace until they were fully recovered.
"The standard public health is 48 hours after the last episode of either diarrhoea or vomiting," he said.
"So we would certainly encourage parents if their children have been sick or had diarrhoea over the weekend to keep them off school until they've had that clear 48 hours.
"It is very infectious and we want to try to avoid passing it to any other children."
A Dumfries primary school has reopened after a sickness bug among a "high number" of pupils and staff forced its closure.