The World Health Organization has announced the mosquito-borne Zika virus is expected to hit the U.S. as it spreads across the Americas. Only Canada and Chile are likely to be spared, the agency says, because the type of mosquito known for spreading the illness is not present in either country. The disease, which can severely harm unborn babies if the mother is infected, has been reported in at least 21 countries. Pregnant women have already been asked not to travel to Bolivia, Brazil, and Colombia, among many other countries. The Centers for Disease Control and Prevention have said pregnant women who have traveled to South and Central America should be monitored for the virus.