Does the Zika virus really cause the birth defect microcephaly? From Brazil's Amazon Basin to the favelas of Sao Paulo, researchers are looking for answers. We hear what's being learned on the ground about the deadly mix of over-crowding, extreme poverty, drought and climate change. They're combining to foster the breed of mosquitos that carries Zika as well as other, deadly diseases. The World Health Organization's declared a "public health emergency" — but is also telling athletes and fans of the summer Olympics not to worry.