Scarlet fever is now known to have affected pupils at six Dundee schools.
The city council education department confirmed that Baldragon Academy, Forthill Primary and Longhaugh Primary have had cases.
It is understood there have also been cases at Rowantree Primary.
Officials had previously acknowledged that the illness had affected pupils at Downfield Primary, where at least 20 children came down with the bug, and Craigiebarns Primary.
Scarlet fever stopped being a notifiable disease in Scotland four years ago, so there is no national data on the number of cases.
However, Public Health England said the number recorded south of the border during February was twice the long-term average and had reached a 24-year high.
Scarlet fever tends to affect younger children, which is why most of the affected schools to date have been primaries, but older children such as those at Baldragon can catch it too.
A council spokesman said: “In line with medical advice, the school has issued a letter to all parents and families.
“They have been given guidance about contacting a health professional if they think their child has scarlet fever.
“The secondary’s communication also stresses that any pupil with the condition should not attend school for at least 24 hours after they take their first medication.”
Former councillor Allan Petrie claimed the council could have done more to warn parents at Rowantree Primary.
He said: “About a week ago we suddenly realised there had been an outbreak of scarlet fever at the school.
“But I only found that out by speaking to the boy’s parents and then the school confirmed it.
“But the education department have done absolutely nothing.
“I had to speak to them and even then they said that it was only their responsibility to contact the health authority, not the parents.
“It is ridiculous. They’re happy to tell you about hair lice outbreaks but not a serious thing like this.”