Hi Davin - no big answers, but it does seem that there are different strains of SARS, some much more virulent/infectious than others. As Scott mentioned below, there may also be synergistic effects of other medical conditions.

One difference between Canada and the US is that (from the WHO site : "Due to differences in the case definitions being used at a national level, probable cases are reported by all countries except the United States of America, which is reporting suspect cases under investigation."

In other words, by the more commonly used definition, the US likely would have a lot fewer cases in it's entry. I know that there are at least 2-3 times as many suspected cases in Canada that there are probable or confirmed cases. The patient in my own hospital was "suspected" (since found not to be SARS); they were not counted among the reported Canadian cases, but would have been in the US.

From WHO: "Health Canada has received reports of 217 probable or suspect cases of SARS. There have been 9 deaths in Canada. All Canadian cases have occurred in persons who have travelled to Asia or had contact with SARS cases in the household or in a health-care setting. The largest outbreak has occurred in Ontario, where 87 probable and 92 suspect cases have been reported. British Columbia is reporting 3 probable and 23 suspect cases, New Brunswick is reporting 2 suspect cases, Saskatchewan is reporting 1 suspect case, Alberta is reporting 5 suspect cases, and Prince Edward Island is reporting 4 suspect cases." For that date, the tallied number of cases in Canada was 90, less than half the total of probable plus suspected.

(another case of Us exceptionalism? but I digress...)

The current WHO tally is 2671 cases, 103 deaths - compare to 10 days ago, 1550 cases, 54 deaths.

I don't know why there have been no deaths reported in the US - a good thing certainly. Conspiracy people might say that deaths are there but not reported in order not to cause panic, or not to distract citizens from the Iraq show.