It's feared a Dunedin university student with measles may have been in contact with numerous people.
The 18-year-old woman living in a flat came down with the illness this week. It's believed she had not been vaccinated.
Southern District Health Board Medical Officer of Health Dr Susan Jack says they're on high alert because of orientation week, during which students like to party and socialise before the pressures of uni work begin to weigh them down.
"You can be infectious before you've got any of the symptoms," she told Newshub. "Students are very social beings, so there is potential for it to spread."
A rash usually appears about two to four days after the first symptoms, often a runny nose, dry cough and high temperature.
"People are infectious from five days before the onset of the rash to five days after the rash starts and should stay in isolation during this time," says Dr Jack.
"This means staying home from school or work and having no contact with unimmunised people.
"Measles is highly infectious. If you think you have symptoms of measles - it is vital that you do not visit your GP office, emergency room or after hours clinic. Instead, please phone your GP practice or Healthline for advice."
Anyone with symptoms is asked to stay home and call a doctor.
Healthline's number is 0800 611 116.
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Measles can be prevented by receiving the MMR vaccine, which is available through your GP. People who've had two doses, or born before 1969, are considered immune.