Infectious disease specialists are particularly focused on the potential for Zika to spread to the United States. As many as 2,00,000 Americans are expected to travel to Rio de Janeiro for the Olympics in August. When they return to the Northern Hemisphere and its summer heat, far more mosquitoes will be around to potentially transmit the virus in the U.S.

Brazilian researchers say they believe that Zika, which has been linked to severe birth defects, came to their country during another major sports event — the 2014 World Cup — when hundreds of thousands of visitors flowed into Brazil. Virus trackers here say that the strain raging in Brazil probably came from Polynesia, where an outbreak was rattling small islands around the Pacific.