A sixth case of measles has been diagnosed in Niagara Region, bringing to 17 the total number of cases of the highly infectious disease confirmed in Ontario since the end of January.

The latest ill patient is an unvaccinated female who is connected to the first person in Niagara to have been diagnosed with the measles earlier this month, as are all the cases that have been confirmed there so far.

"It is therefore not a sporadic event," said Valerie Jaegar, the medical officer of health for the region, which includes the cities of Niagara Falls and St. Catharines.

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Dr. Jaegar said she would not be surprised if the virus continues to spread in the province.

One of the Niagara patients, a 14-year-old girl, attended a Christian youth event called "Acquire the Fire," at the Queensway Cathedral in Toronto on Feb. 6 and 7, prompting a warning for more than 1,000 participants to be on the lookout for the symptoms of measles, including fever, dry cough, runny nose and a telltale rash that begins on the face and moves down the body.

"Measles is a highly contagious disease," Dr. Jaegar said Wednesday. "I think it's certainly reasonable to say in Ontario that we have no evidence that the outbreak is finishing. We may well be in the early stages of the outbreak. I don't know that there will be more cases, but I certainly wouldn't be surprised. It certainly doesn't mean that things are out of control. It just means that it's measles."

All of the patients in the Niagara outbreak are under the age of 30. The first patient in the cluster travelled to Toronto twice at the end of January, the same period when the first handful of measles cases was diagnosed in Canada's largest city.

So far, nine cases of the measles have been diagnosed in Toronto this year. Two more were confirmed just outside the city – one in York Region, another in Halton Region.

Unlike in the Niagara cluster, no clear links have been established between the 11 patients in the Greater Toronto Area through interviews with the patients or their parents, leaving public health officials scratching their heads over where the outbreak originated and how, precisely, it spread.

The story is different in Quebec, where 10 recently confirmed cases of the measles have been traced back to an outbreak that began at Disneyland in California. As of Feb. 13, the U.S. Centers for Disease Control and Prevention reported 141 cases of measles in 17 states, most linked to the Disneyland outbreak.

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Last year, more than 400 cases of measles were diagnosed in British Columbia's Fraser Valley linked to a Christian school with a low vaccination rate. In 2011, more than 750 cases were diagnosed in Quebec. In most other years in the past decade, fewer than 100 cases have been confirmed annually across the country, according to the Public Health Agency of Canada (PHAC).

In Toronto, measles remains rare: Just 14 cases were reported between 2009 and 2013, the last year for which complete statistics are available.

Canada's overall vaccination rate against the measles is high – about 95 per cent – but the coverage rate is much lower in some communities that eschew immunization for religious or philosophical reasons.