A Perth school student has returned from overseas with measles, sparking a citywide health alert.
While still infectious, the secondary student attended Perth Waldorf School in Bibra Lake after which a "substantial number" of pupils were identified as not having been vaccinated, the WA department of health said on Friday
Waldorf, which takes pre-school up to secondary enrolments, has a metropolitan-wide catchment area, prompting the warning for the entire Perth area.
The health department says it is likely there will be more cases of the highly contagious disease over the coming weeks.
Director of Communicable Diseases, Dr Paul Armstrong said "approximately nine out of 10 susceptible persons in close contact with a measles patient would develop it".
"The illness is spread by tiny droplets, released when infected people cough and sneeze," he said.
Early symptoms include fever, cough, runny nose and sore eyes with a red blotchy rash developing about three days later.
Anyone who thinks they are infected with measles should telephone their GP surgery or emergency department so they can be isolated on arrival, the department said.