Health officials are urging Torontonians to check their vaccination records after the city confirmed its sixth case of measles.

St. Joseph's Health Centre in the city's west end said Friday a patient who was in its emergency department from Jan. 27 to 29 has been diagnosed with the disease.

Two children, both under two years of age, and three adults in the Toronto area were also diagnosed with measles earlier this week.

The latest patient is an adult who was born before 1970, according to a statement from Toronto Public Health. It's significant because those born before 1970 are presumed to have acquired a natural immunity to measles.

There is no known connection between the cases which, officials say, indicates measles is "currently circulating" in the city.

Another case of measles was confirmed earlier this week in Ontario's Niagara region.

St. Joseph's said it worked with public health officials to notify those who may have been exposed to the patient.

One such letter, obtained by CBC News, warns the recipient that the highly infectious disease causes high fever; cold-like symptoms including a cough and runny nose; and a red rash, usually all over the body, lasting for between four and seven days.

It says to watch for symptoms until Feb. 19. People can spread the disease before symptoms have appeared.

Separately, in a letter to parents on Friday, officials said if there is a measles outbreak at a Toronto school, children without proper vaccination "will not be allowed to attend school until the outbreak is over."

More cases

Officials say residents should check their immunization records to make sure they are fully protected.

"We have people who are about 35 to 40 that probably only received one dose," of the vaccine, said Shelley Deeks of Public Health Ontario.

We're seeing … people who have declined the vaccination getting sick. - Dr. Brett Belchetz

"So there is a differing level of susceptibility."

Deeks says she suspects there will be more cases.

Two doses of MMR vaccine provide full protection against mumps, measles and rubella for over 95 per cent of people, according to Toronto Public Health.

Officials say anyone unsure of their immunizations should contact their healthcare provider.

There are no confirmed measles cases at St. Joseph's, according to the statement from Dr. Mark Downing, head of the infectious diseases division.

Children at risk

Prior to the development of a vaccine, measles was responsible for 50 to 75 deaths per year in Canada, according to Immunize Canada. Since then, the number of cases has fallen by more than 99 per cent.

Measles can cause rash, fever and more serious complications including encephalitis. (U.S. Centres for Disease Control and Prevention) However, among those without vaccinations, one case of measles usually manages to infect another 16 people, according to emergency room physician Dr. Brett Belchetz.

"When we look at this outbreak we're having right now now, we're not seeing a failure of the vaccine. We're seeing … people who have declined the vaccination getting sick because the bug is in the community," he said.

The disease also poses a particular risk to children. It can cause a rash, fever and can lead to encephalitis — an inflammation of the brain tissue.

"Even if you survive that, there are often major life-long neurological complications," Belchetz said.

Last year, five provinces reported cases of measles. In 2011, Quebec saw more than 700 cases but no deaths. Quebec officials are currently waiting on test results and warning the public of a possible outbreak.