Zika virus 'spreading explosively

The Zika virus is "is now spreading explosively" in the Americas, the head of the World Health Organization said Thursday, with another official estimating between 3 million and 4 million Zika infections in the region over a 12-month period.

"The level of concern is high, as is the level of uncertainty," WHO Director-General Dr. Margaret Chan told her organization's executive board members. "We need to get some answers, quickly."

The lack of any immunity to Zika virus and the fact the mosquitoes that spread it can be found most "everywhere in the Americas" -- from Argentina to the southern United States -- "explains the speed" of the Zika virus's spread, explained Dr. Sylvain Aldighieri, an official with the WHO and Pan American Health Organization.

Aldighieri gave the 3 to 4 million estimate for Zika infections (including people who do not report clinical symptons) based on data regarding how a different mosquito-borne virus -- dengue -- spreads. He cautioned that's only an estimate, while acknowledging the virus is currently circulating with "very high intensity."

Some 80% of those infected with Zika virus never know they have it. Still, there are major worries about dangers facing pregnant women and their babies.

Chan explained that, where the virus has arrived, there's been a corresponding "steep increase in the birth of babies with abnormally small heads and in cases of Guillain-Barre syndrome." Having small heads can cause severe developmental issues and sometimes death. Guillain-Barre, meanwhile, is a rare autoimmune disorder that can lead to life-threatening paralysis.

Dr. Bruce Aylward, another WHO leader, cautioned that no definitive link has been established. But, he said, there's legitimate reason to be concerned.

And there's no doubt that Zika potentially poses a dire health to areas that are home to millions of people.

What is far less clear is what to do about it.