Update (8/1/2016): On Monday, Florida Governor Rick Scott announced that 10 new Zika cases have been identified in Miami's small, localized outbreak, bringing the total to 14 cases. All of the new cases, thought to be transmitted by local mosquitoes, appear to have occurred in the same one-square-mile area that officials had previously identified as an area where active transmission may be occurring.

In six of the 10 new cases, the individuals had no symptoms of a Zika infection and were only found during a survey of residents. No mosquitoes surveyed have yet to test positive for carrying Zika virus. Gov. Scott is now calling on the Centers for Disease Control and Prevention to activate emergency response efforts. Also today, the United Kingdom issued a Zika travel advisory for Florida. Our original story on the initial discovery in Florida appears below.

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 area of Miami-Dade County, just north of downtown.

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.

Health experts and officials had expected that such small, localized outbreaks would occur in the US during summer months when mosquitoes are active, particularly in places like Florida. Health officials in the state are continuing to monitor and spray for mosquitoes, and they are also testing residents in the affected and nearby areas for undiagnosed cases.

Zika often causes mild to symptom-less infections in healthy adults. However, when the virus infects pregnant women, it’s known to cause severe birth defects, such as a small and malformed brain and head, a condition called microcephaly. Zika is also linked to paralyzing conditions, such as Guillain-Barré syndrome.

Earlier this week, the FDA advised Miami-Dade County, Broward County, and adjacent counties to stop accepting blood donations until donors could be screened or donated blood could be treated to kill any potential contamination from Zika virus.

The Florida Health Department is giving out Zika prevention kits and advising residents to remove any standing water in their yards and homes, which is where mosquitoes breed.