A female Aedes aegypti mosquito dines on the arm of a researcher at the Biomedical Sciences Institute of Sao Paulo University. (Andre Penner/AP)

In Florida, 328 people have been infected with the mosquito-borne Zika virus, an infection that causes only mild illness in most people but has been linked to severe brain and birth defects in newborn babies.

Until recently, all U.S. cases of Zika, including those in Florida, were linked to people who traveled to countries in Latin America and the Caribbean that had outbreaks.

That changed this month, when health officials in Florida began investigating a possible non-travel-related case in Miami-Dade County. Then another popped up in neighboring Broward County. The South Florida counties have the most cases of Zika statewide.

On Wednesday, the Florida Department of Health added two more possible non-travel-related cases to their list, bringing the total to four and suggesting that mosquitoes in the continental United States may be infected with the disease.

“Evidence is mounting to suggest local transmission via mosquitoes is going on in South Florida,” Tom Skinner, a spokesman for the Centers for Disease Control and Prevention, told Reuters.

Florida Health Department officials are urging residents and visitors to cooperate in the investigation, which began Wednesday, and will include door-to-door outreach and collection of urine samples. Epidemiologists will survey people who live within a 150-yard radius — the flying range of the mosquitoes that carry Zika — of the infected person, Reuters reported.

“These results will help the department determine the number of people infected,” the Florida Health Department said in a news release.

Zika causes a rash, fever and joint pain in most people, according to health officials, but CDC researchers have determined that the virus leads to microcephaly in infants, a birth defect in which a baby’s head and brain are smaller than normal. Microcephaly impairs cognitive function, according to the CDC, and has been known to cause seizures, hearing loss, vision problems, difficulty swallowing, developmental delays, speech defects and intellectual disabilities.

In June, the Florida Health Department announced the state’s first confirmed case of microcephaly in an infant born to a mother with Zika. She contracted the virus while she was in Haiti, according to the health department.

Florida is tracking 53 Zika cases involving pregnant women.

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