Five confirmed cases of measles have been reported in the Rockingham area in Perth's south.
Public health authorities are in the process of providing information to people who were potentially exposed to offer treatment and immunisations if appropriate.
But they say it's not possible to identify people who may have been exposed in public places.
Acting Director of the Communicable Disease Control Directorate, Paul Effler, said measles was a serious and highly contagious virus spread by tiny droplets released when infected people cough and sneeze.
"Every measles case is treated as a public health emergency because of the risk of local spread," Dr Effler said on Tuesday.
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"With high vaccination coverage, naturally occurring measles has been eliminated from WA for around 20 years, but occasional cases and small outbreaks still occur, usually associated with tourists or WA residents who are infected overseas."
People with measles typically develop symptoms about 10 days after being exposed.
Early symptoms include fever, cough, runny nose and sore eyes, followed by a red blotchy rash three or four days later.
Dr Effler said anyone who believed they may have been exposed and who develops a fever and other symptoms should see a doctor.
