Zika burst into the international news this year, along with travel alerts, heartbreaking images of children with birth defects and a link to an autoimmune disease that can cause paralysis. The virus first surfaced in this country back in 2007, when an American medical volunteer contracted the disease during an outbreak in Micronesia and then became sick with it back in Alaska. Since then, more than 50 cases have been identified in the U.S. Almost every one of these patients contracted Zika while abroad, but at least two infections were acquired via sexual contact.

Because the U.S. Centers for Disease Control and Prevention does not provide detailed state-by-state breakdowns of Zika cases, Scientific American gathered and analyzed information from the health departments of all 50 states and the District of Columbia and followed up with some county and city health officials. The result is this exclusive map of how the virus first made its way to the U.S.

SOURCE: U.S. CENTERS FOR DISEASE CONTROL AND PREVENTION (countries with active transmission); Map by Mapping Specialists, Graphic by Jen Christiansen