The World Health Organization says, "alarm is high." But so is "uncertainty." In the next year, the Zika virus might infect three to four million people. But nobody knows for sure if the mosquito-borne illness really is responsible for birth defects in South American babies. Women there are being warned about pregnancy, and the WHO today called an emergency. US health officials say an outbreak here is unlikely. We hear from Brazil, where fear is on the rise, and get the latest on controlling mosquitos.