Why the United States is so vulnerable to the alarming spread of Zika virus

What is far less certain, say public health and infectious disease experts, is Zika’s potential reach and impact here. The South is seen as vulnerable because of its warm, humid climate and pockets of poverty where more people live without air-conditioning or proper window screens. Plus, the region is already home to mosquitoes that can transmit the virus.

Some models estimate as many as 200 million people live in areas that might be conducive to the spread of Zika during summer months — including the East and West coasts and much of the Midwest. That makes for a huge target as researchers scramble to determine exactly how the virus manifests itself in the human body, who is particularly at risk, and why.

Their urgency comes amid reports from Brazil, the epicenter of Zika, of thousands of newborns with a rare condition involving brain damage. Doctors there have also seen a surge in another rare syndrome, Guillain-Barré, which can lead to paralysis. With both, a link to Zika is suspected.