A worker in Brazil disinfects the famous Sambadrome as part of the fight against the mosquito Aedes aegypti, which spreads the Zika, dengue, and chikungunya viruses. Marcelo Sayao/ZUMA The outbreak of Zika virus in Latin America is “spreading explosively,” the director of the World Health Organization warned at an emergency meeting in Geneva on Thursday. Last week, the epidemic took a surreal turn when health officials in El Salvador advised women there not to get pregnant for the next two years. Similar, though less extreme, warnings have been issued by Brazil, Colombia, and several other countries. The virus has infected more than 1 million people during the current epidemic, and health officials say it may be linked to a spike in microcephaly, a rare condition in which infants are born with unusually small heads.Behind the outbreak is a complex combination of environmental and economic factors. Here’s what you need to know: