This suggests the possibility that local transmission of the mosquito-borne virus that has been linked to severe birth defects has begun, officials said.
Zika infection poses significant risks to pregnant women, because it can cause the birth defect called microcephaly, which results in babies born with undersized heads and underdeveloped brains.
But, the virus poses little threat to most other people, with about 80 percent of those infected never noticing any symptoms.
The four cases are in the South Florida counties of Miami-Dade and Broward.
"Evidence is mounting to suggest local transmission via mosquitoes is going on in South Florida," said Tom Skinner, a spokesman for the U.S. Centers for Disease Control and Prevention, CNN reported. "These cases fit similar transmission patterns (of) mosquito-borne diseases like chikungunya that we've seen in South Florida in years past."
CDC officials have said repeatedly they expect to see cases of local transmission of the Zika virus this summer in warm, humid southern states such as Florida, Louisiana and Texas. The virus is typically transmitted through the bite of Aedes mosquitoes.
So far, the 1,400 infections reported in the United States have been linked to travel to countries with Zika outbreaks in Latin America or the Caribbean. Brazil has been the epicenter of the Zika epidemic to this point.
In addition to mosquitoes, the Zika virus can be transmitted through sex. The CDC has reported 14 cases of sexually transmitted infections. These infections are thought to have occurred because the patients' partners had traveled to countries where Zika is circulating, the CDC said.
The Zika virus also has been linked to a rare paralyzing condition called Guillain-Barre syndrome.
The CDC advises pregnant women not to travel to an area where Zika transmission is ongoing, and to use insect repellent and wear long pants and long-sleeved shirts if they are in those areas. Partners of pregnant women are advised to use a condom to guard against sexual transmission during pregnancy.
The U.S. Centers for Disease Control and Prevention provides more information on mosquito-borne diseases.
This Q&A will tell you what you need to know about Zika.
To see the CDC list of sites where Zika virus is active and may pose a threat to pregnant women, click here.
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