(credit: CDC)

On Friday, Florida health officials confirmed the first local Zika outbreak in the continental US. The outbreak so far involves four people believed to be infected by bites from local mosquitoes, and health officials believe that active transmission may be occurring in a one-square-mile-wide area of Miami-Dade County, just north of downtown.

The area of Miami-Dade county where officials believe mosquitoes may be actively transmitting Zika. (credit: Florida Department of Health)

So far, mosquito surveillance has not detected the presence of Zika virus in any mosquitoes in that area—or any other areas. However, the four infected Floridians, residents of Miami-Dade County and bordering Broward County, had not traveled to a Zika-stricken area or recently had sex with an infected or potentially infected person. By ruling out those two routes of transmission—which explain how 1,657 other US residents became infected—officials believe local mosquito transmission is to blame.

Local mosquitoes presumably picked up the virus from returning travelers and passed it on to uninfected residents. So far, Florida has documented 381 cases statewide.