WHO declares Zika virus 'extraordinary' emergency

Agency: There could be 4 million cases in the Americas next year

Concern over the Zika virus linked to birth defects in the Americas widened across the world on Tuesday, as several nations identified new cases or announced a state of emergency in the wake of the World Health Organization's declaration of a global health crisis over the virus' explosive spread.

On Monday, the UN health agency called the continuing spread of the disease an "extraordinary event" that poses a global public health threat.