But don't you think that if it were common for people with serious illnesses to come to the US, we'd see much more made of it than we do from those with a large vested interest in noting the influx?
Here's a study that puts the numer of Canadians using the US healthcare system at about 0.61%: http://content.healthaffairs.org/content/21/3/19.full.html
As for that study, I'm a little concerned that they only looked at a handful of states, and this little line at the bottom:
"The authors acknowledge financial support from the Canadian Institutes of Health Research (formerly the Medical Research Council of Canada) for this research."
My guess: both your source and mine are biased and flawed, and the truth lies somewhere in the middle.