Something fascinating has apparently been preventing the spread of the Zika virus in Florida this year: herd immunity, which is when enough people in an area are infected with a virus and develop a resistance to it. Scientists think that a decline in outbreaks in the Caribbean and South America, the epicenter of the disease, may be slowing it down in Florida and preventing its spread in the United States.

“People that were infected before can’t be infected again. That’s our understanding,” Dr. Henry Walke, the U.S. Centers for Disease Control and Prevention’s incident manager for Zika response, said in a report in the Miami Herald. “So you don’t have as much of the virus circulating. That’s true not only in Puerto Rico but throughout the Caribbean and throughout South America.”

But that won’t necessarily stop the virus from re-emerging in the United States, which has happened in Florida before with other mosquito-borne viruses like chikunguny and dengue. As a result, authorities know they must be vigilant.

“CDC has issued guidance for travel, prevention, testing, and preconception counseling related to risks for pregnant women and couples considering conception in areas of active Zika virus transmission in the continental United States and Hawaii,” the CDC says on its website. “CDC has identified two types of geographic areas to describe where Zika virus-related domestic guidance applies: Zika active transmission areas (designated as red areas) and Zika cautionary areas (designated as yellow areas). The recommendations in the tables below are based on the CDC Zika Interim Response Plan, May 2017.”