i'm no expert, so plz not throw bricks on me
the diagnosis procedure of the specific strain is much more complicated than SARS, and more time consuming, more money consuming. this is part of the reason that mexico didn't confirm or discover this is a new strain until 2 months after the first child reported disease.
http://www.juexiang.comgov/cdrh/emergency/panel-authorization.html
so i guess there are reasons why china haven't find any even possible cases yet.
in this way, america is still a safe place, because of the advanced diagnosis method and treatment, as long as you go to the hospital immediately after any symptoms of flu turn out.
by the way, this time china is sure not the source, but if outbreak of swine flu happens in china, it will be more difficult to control the spread than in mexico.
