This story was originally published by Mother Jones and is reproduced here as part of the Climate Desk collaboration.

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 a million people during the current epidemic, and health officials say that 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:

What is Zika?

Zika was first identified in monkeys in Uganda’s Zika Forest in 1947. In the years since, the disease has slowly migrated eastward around the globe, following oceanic trade routes with the help of infected sailors and mosquitoes trapped in the holds of ships. The first serious outbreak occurred in 2007 in Micronesia, where up to 60 people were infected, followed by cases in French Polynesia and on other Pacific islands. The current outbreak, which started late last year in Brazil, is the most serious yet and the first one in the Americas.