Which ones are those? I'm still waiting for the first good comment myself. Hint: Putting up a 50 foot fence along the border won't stop communicable disease infiltration as long as legal travel occurs across it. Plus, this particular disease isn't known south of the border either: it isn't polio.
Specially modified food shipped North? Shipping from outside via container ships? Specific toys? Too many possibles, imho. I do pray a handle on this outbreak is coming quickly.
Illegal travel is ever present and persistent ... even under the guise of legal travel. Remember, not long ago there was the use of x-ray for a particular color while traveling. With the ever changing processes of nature and weaponry am surprised it has not happened before now. And then ... nothing but nature maybe is taking place but remains unknown. May this mystery be solved soon.
Yours is the first comment to correctly note that polio-like symptoms does not equal polio. Seems most commenters only read the headline.