In the past 10 days four adults and two children in Canada's largest city have come down with the disease.
As well, a woman in the Niagara region southwest of Toronto has also contracted measles.
Toronto Public Health spokesperson Lenore Bromley says the latest person to become infected was born before 1970.
That's significant because that was before measles vaccine was introduced in Canada.
People born before 1970 are generally assumed to be immune to measles because most children contracted the highly contagious virus in the days before vaccine was available.
Bromley says public health is still trying to figure out how the virus was introduced to Canada, whether there are connections among the cases and whether more cases are going undetected.
But she says to date there are no clear links among any of the infected people.
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