As many as 3 million visitors will descend on London this summer for the Olympic Games, bringing with them viruses from around the world -- including the one that causes measles.

That's why infectious disease experts are warning Canadians planning to travel to London for the Games to make sure they are vaccinated.

Measles is not exactly a common illness; in fact, it was once mostly eradicated in Canada. But a huge outbreak last year in Quebec shows just how vulnerable many Canadians are to it.

Last summer's outbreak in the area of Drummondville was by far the worst seen in the Americas in 20 years. A full 776 people were reported ill and one out every nine required hospitalization.

That outbreak was likely started by someone who picked up the disease while traveling in Europe. The Public Health Agency of Canada analyzed virus samples from the infected Quebecers and confirmed they had the same strain that has been circulating through France.

France has been dealing with a number of measles outbreaks in recent years that have spread throughout Europe -- including to London. In all of Europe last year, there were an astonishing 26,000 cases of measles which resulted in eight deaths.

In the U.S., 2011 was the worst year for measles in 15 years, health officials said this week. There were 222 cases-- a big jump from the 60 or so seen in a typical year. Once again, most of those cases were brought in by foreign visitors or by U.S. residents who picked up the virus overseas.

Now, with so many expected to travel to England for the Summer Olympics, infectious diseases experts are worried  that Canadian travellers could once again bring the illness home.

U.S. health officials are urging everyone -- particularly international travellers -- to make sure they're fully vaccinated.

"For those of you travelling abroad, bring back memories and not measles," said Dr. Anne Schuchat of the Centers for Disease Control and Prevention.

Ukraine, which will host the Euro 2012 soccer championship in June along with Poland, and which has grappled for months with a big measles outbreak, is urging fans coming over from aboard to get vaccinated.

"If you plan to come to Ukraine, please get vaccinated at home," the Kiev Post newspaper quoted Oleksandr Kravchuk, whos' the deputy head of the state sanitary and epidemiological service. "...The situation with measles is unfavourable in our country."

Dr. Caroline Quach, an infectious diseases expert at the Montreal Children's Hospital, says she would encourage any Canadian planning to travel in Europe this year to go through their childhood vaccination records and be sure they got the MMR vaccine, which protects against measles, mumps and rubella.

She points out that one doesn't even have to come into contact with an infected person in order to catch measles; they just have to breathe air that they've breathed.

"Measles travels in the air, and it can survive in very, very, very small droplets and travel for kilometres," she told recently from Montreal.

"So it's possible that you will never meet, face-to-face the person who gives you measles."

Quach says measles is one of the most contagious infectious diseases around. To put it in perspective: one person with measles can infect 16 others; a person infected with the pandemic H1N1 flu typically could infect only two others.

The good news is that the vaccine against measles is considered both safe and effective. The bad news is that vaccination rates have fallen throughout Europe – which is what has been allowing these outbreaks to grow.

Just 60 per cent of the population in France is fully vaccinated against measles -- that's far below the 95 per cent rate needed for "herd immunity." France accounted for more than half of all cases reported in Europe in 2011. Other countries with a high number of cases were Italy, Spain, Romania and Germany.

Quach highly recommends that Canadians planning to travel to Europe find out if they are vaccinated against measles.

People born before 1970 in Canada are considered immune to measles, because the disease was circulating so widely then that most people caught it and developed natural immunity.

Those born between 1970 and 1979 likely had one dose of vaccine, which is considered adequate because there was still circulating measles during that period, though its incidence was decreasing.

Canada began recommending in 1996 that everyone get two doses and started a catch-up program to ensure those born after 1979 got two doses. But there may be many Canadians who were missed.

What if you don't know if you've had the shots you need or you can't remember if you were ever infected? Start by going through your health records or asking your parents, Quach recommends. There is also a blood test that can check for measles antibodies, though she warns that the results take a long time to come back.

But there is no danger to getting an extra dose of the vaccine, even if you don't need it, Quach says. Pregnant women are one key exception, though, since the measles vaccine uses a live, though very weakened, virus. Others with immunity problems should check with their doctors about whether they should get the shot.

Most adult Canadians have never seen anyone infected with measles, but Quach will tell you it's not pretty. In the best case scenario, she says, a healthy person who picks up measles will get a fever, cough, runny eyes a rash and be in bed, miserable, for a week.

"But if someone comes near you who has a weak immune system -- maybe someone in chemo, or they're elderly -- then they're at risk of very severe measles," says Quach.

That can lead to such complications as deafness, encephalitis and death. What's more, there is no approved treatment for measles; all that doctors can do is manage the symptoms.

Quach recognizes that there are certain people who distrust vaccines and who are worried about getting vaccinated. But she recommends doing it for the sake of others.

"Refusing to get vaccinated puts yourself at risk but you can always decide for your own life," she says. "However, you're also putting at risk everyone else you encounter."

Follow Angela Mulholland on Twitter: @angemulholland