The World Health Organization says it expects the Zika virus to spread to every country in the Western Hemisphere except Canada.

It says the virus has already "spread to 21 countries and territories of the Americas."

"Canada is off the list simply because it's too cold for the type of mosquito that transmits the Zika virus," NPR's Jason Beaubien reports to our Newscast unit.

The illness caused by the virus has been blamed for birth defects.

The WHO's regional office — the Pan American Health Organization — says the type of mosquito provides one explanation for the virus's rapid spread.

The virus "will likely reach all countries and territories where Aedes mosquitoes are found," PAHO says in a statement.

The organization attributes another factor to the spread of the virus: "The population of the Americas had not previously been exposed to Zika and therefore lacks immunity."

Additionally, some epidemiologists think the virus may have mutated. "This may be a new strain that's traveling very quickly but we really don't know," Yale epidemiologist Albert Ko tells Jason.

Jason says the virus was first identified in Brazil in May 2015 and "coincided with a marked spike in severe birth defects, and possibly other neurological problems."

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