Four cases of the virus being transmitted locally by mosquitoes in Florida have officially been confirmed.

According to Florida health officials, these are the first cases of mosquitoes transmitting the virus within the continental United States. It has been concluded that these are mosquito-transmitted instances because the infected individuals have no other known risk of exposure as none have traveled to any Zika-affected areas or engaged in sexual contact with an infected person.

State officials believe the area of active transmission is limited to one spot and have identified a one-mile area north of downtown Miami. Although mosquitoes within the area have been tested, according to governor Rick Scott, none carry the virus. Officials are also canvasing neighborhoods to collect urine samples of residents to test for infection.

“While no mosquitoes have tested positive for the Zika virus, [the health department] is aggressively testing people in this area to ensure there are no other cases,” Scott said in a statement. “If you live in this area and want to be tested, I urge you to contact the county health department.”

Currently, there are more than 1,300 Zika cases in the continental United States, but up until now, all had been the result of travel or sexual contact. The latest infected individuals, three males and one female, have not been hospitalized and are not experiencing severe symptoms.

According to PLOS, cities predicted to have the highest risk for a Zika epidemic include New York City; Philadelphia; Washington DC; Atlanta, Charleston, SC; Savannah, GA; Miami; Tampa, FL; Orlando, FL; Jacksonville, FL; Tallahassee, FL; Mobile, AL; New Orleans, Houston, Brownsville, TX; Dallas, and Los Angeles.

This mosquito-borne illness poses a major public health threat. While side effects are generally mild and include flu-like symptoms or a rash, moms-to-be who become infected can transmit the condition to their unborn babies. This can lead to microcephaly, a congenital disease characterized by a small head and incomplete brain development. However, not much is known about the extent of this risk, and researchers are closely monitoring 279 pregnant women infected with the Zika virus across the United States and US territories.