Factbox: Why the Zika virus is causing alarm in the Americas

(Reuters) - Global health officials say the Zika virus, linked to severe birth defects in thousands of babies in Brazil, is spreading rapidly in the Americas and could infect 3 million to 4 million people. The virus is transmitted to people through the bite of infected female Aedes mosquitoes, the same type of mosquito that spreads dengue, chikungunya and yellow fever. The Pan American Health Organization (PAHO) said Aedes mosquitoes are found in all countries in the Americas except Canada and c