A baby is one of six people to contract measles after coming in contact with an infected overseas traveller in Victoria.
The tourist from Vietnam attended family events in Mornington and St Kilda on May 4 and 5 before visiting Moonee Ponds, North Melbourne, Coburg and Mount Waverley.
It is the largest linked cluster of measles in Victoria since March last year, when nine people developed the illness, the health department confirmed.
The people are believed to know to each other because they attended the same events, acting Chief Health Officer Angie Bone said on Wednesday.
"All of these cases have had fairly extensive contact with the community in Victoria in a number of different suburbs," Dr Bone said, urging others to watch for symptoms.
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Those six people visited Geelong, East Malvern, Chadstone, Noble Park, Hawthorn, Oakleigh, Mount Waverley, Fairfield, South Wharf, Armadale, Abbotsford, East Melbourne, Baxter, Frankston, Karingal and St Kilda Road while infectious.
More than 30 babies and toddlers who had contact with the child - who was too young to be vaccinated - at a childcare facility were given antibodies on Wednesday to prevent them developing the disease, Dr Bone added.
In a separate case, a person who was diagnosed with measles this week after visiting Bendigo and Echuca is linked to another overseas traveller who visited in April.
"It is quite likely, despite our best efforts, that we will have more cases," Dr Bone said.
Symptoms of the disease include sore eyes, a runny nose, cough, rash and fever. In the worst instances, measles can lead to death.
Twenty-two people have been diagnosed with measles in Victoria this year.
