At least two children in the Wellington region have been diagnosed with the rare infectious disease scarlet fever in recent weeks.
Karori Normal School alerted parents on Wednesday that one child was confirmed to have the illness and a Lower Hutt preschool reported a case in April.
The regional medical officer of health, Annette Nesdale, said scarlet fever, which caused deadly outbreaks before the discovery of antibiotics, was not a notifiable disease, so it was not known how many cases occur.
She said the disease typically began as a sore throat and parents should seek medical attention for sick children.
Scarlet fever increasingly rare
An infectious disease expert says it is not fully known why scarlet fever has largely disappeared from the modern world, nor why it persists in some areas.
Dr Tim Blackmore from the Capital and Coast District Health Board said the disease caused deadly epidemics before the advent of antibiotics.
He said it was caused by a reaction to a toxin produced by a bacteria Streptococcus or Staphylococcus infection that has been infected by a virus.
Dr Blackmore said scarlet fever, which caused deadly outbreaks before the discovery of antibiotics, has declined as people have developed immunity to the rash-causing toxin.
"People develop antibodies to these toxins so that they're immune to the rash even if they're not necessarily immune to the bug itself, so sometimes you can have an outbreak of it in a childcare centre or rest home or something like that, but they've become increasingly rare."
Dr Blackmore said scarlet fever was now an old-fashion disease that mainly infected children in poor, over-crowded households.