Public health officials in Toronto are investigating four confirmed cases of measles in the city, involving two children under 2 years of age and two adults from different families.

Toronto Public Health says it appears that the cases are not connected and the families had no contact with one another. That suggests that whoever infected the four has not yet been identified.

"At this point in the investigation, no source case has yet been identified and there are no known links or contact between the cases," the agency said in a statement Monday.

Toronto Public Health added that it is important that the public know that measles is circulating in Toronto.

According to infectious disease specialist Dr. Neil Rau, those who are infected likely came into contact with someone who had measles and likely had no idea. He or she also likely came to Canada from abroad.

Measles is widespread in many parts of the world, including India, sub-Saharan Africa, the Philippines, and South America, Rau said.

“When people from those countries arrive here, they won’t necessarily have symptoms, they’ll be incubating the disease, and they can be contagious four days before coming out with the rash,” Rau told CTV News Channel on Monday afternoon.

“Then they become contagious, get a rash and get over it, but they can spread it to many people in the process if they don’t have immunity.”

While most people in Canada have received the vaccine, there is particular concern for children under the age of one, who are not yet eligible to receive the vaccine.

As well, people born between 1970 and 1996 only received one dose of the vaccine, and have not had any exposure to the virus, Rau said.

“So we speak of something called waning immunity in people born between 1970 and 1996 who can come down with a more modified form of measles,” he said.

He noted, however, that it is not uncommon to see sporadic measles cases in the city.

The Public Health Agency of Canada and Toronto Public Health recommend the following measures:

Check your immunization record or with your health care provider to make sure you and family members are up to date with the measles vaccination (MMR or MMRV).

If you have received two doses of measles-containing vaccine, then you are at very low risk of catching the disease. (Those born before 1970 are considered protected against measles.)

If you are travelling, ensure your immunizations are up to date before you leave, including family members travelling with you. This is very important if you are planning to travel to the United States, which is currently experiencing outbreaks of measles in several states.

If you suspect you might have measles, do not go to any medical office or facility without informing them that there is a possibility you have been infected.

Watch for symptoms of measles. These include a high fever, cold-like symptoms (cough/runny nose), sore eyes or sensitivity to light and a red rash lasting four to seven days. Infants under one year of age, pregnant women, and people with weakened immune systems can get very ill with measles. Those showing symptoms should call their health care provider.

Measles is a highly contagious disease and spreads easily in the air.

There is no specific treatment. Symptoms are usually treated with fluids and medication to reduce fever. Most people fully recover but in rare cases, the disease can lead to encephalitis and death.

The best protection against measles, Rau said, is the vaccine.

“The biggest issue is that people who are opposed to vaccination remember that these childhood immunizations, which include the measles, mumps, German measles vaccine, are extremely important, incredibly effective, completely safe and if they are routinely given it’s very difficult for measles that’s imported from abroad to pick up steam here,” he said.

In comments made to the Empire Club of Canada about Ontario's Action Plan for Health Care, the province's Health Minister Dr. Eric Hoskins, suggested that parents who don't get the measles vaccines for their kids are endangering others.

But Hoskins, who studied measles immunization among children for his PhD, also said he wouldn't move to ban unvaccinated children from school.

Have questions or concerns about measles? Join our live Q & A with infectious disease specialist Dr. Neil Rau at 12 noon ET Tuesday Feb. 3: