(CNN) Four individuals in Miami-Dade and Broward counties have been infected with the Zika virus by local mosquitoes, Florida health officials said Friday.

These are the first known cases of the virus being transmitted by mosquitoes in the continental United States.

"While no mosquitoes trapped tested positive for the Zika virus, the department believes these cases were likely transmitted through infected mosquitoes in this area," according to a statement from the Florida Department of Health.

"As we anticipated, Zika is here," Dr. Tom Frieden, director of the Centers for Disease Control and Prevention, said Friday, adding that "all the evidence we have seen indicates that this is mosquito-borne transmission that occurred several weeks ago."

Frieden explained that confirming local mosquito-borne transmission of the virus is not as easy as confirming the virus in an individual by running a test. He also said that finding a Zika-carrying mosquito is like finding a needle in a haystack, and doing so is not necessary to confirm local mosquito transmission.

Officials believe the local transmission is confined to a small area north of downtown Miami within a single ZIP code. However, local, state and federal health officials are continuing their investigation, which includes going door-to-door to ask residents for urine samples and other information in an effort to determine how many people may be infected. Additional cases are anticipated.

It is possible that someone could have Zika without knowing, since 80% of those infected have no symptoms. When symptoms occur, they can include fever, rash, joint pain and red eyes, and they can last from a few days to about a week.

There is no treatment or vaccine for Zika. None of these four unidentified patients, which include a woman and three men, has needed hospitalization.

To date, there have been 386 cases of Zika in the state of Florida, including 55 pregnant women. The counties with the highest number of cases are Miami-Dade with 99 and Broward with 55.

Blood donations tested

The main way people become infected with the virus is through the bite of an infected mosquito. The female Aedes aegypti and its sister, Aedes albopictus, are the primary vectors, but people can also become infected through other methods, including sexual transmission . Babies can become infected in utero, and there are confirmed cases of transmission from blood transfusion and laboratory exposure

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OneBlood, which handles blood collection in the affected area, began testing all donations for the Zika virus Friday, according to Susan Forbes, OneBlood's vice president of marketing and communications. Any donations that tests positive will be discarded, and the donor will be notified.

OneBlood is the sole blood provider in South Florida and handles most blood collection statewide. It also works in southern Georgia, Alabama and South Carolina.

More than 60 countries and territories are reporting local transmission of the virus. U.S. health officials had warned that there would be local transmission of the virus from mosquitoes but don't expect it to be widespread, as has been seen in Puerto Rico and throughout the Americas. That's based on outbreaks of two similar mosquito-borne diseases, dengue fever and chikungunya.

The reason is largely because of living conditions, including mosquito-control efforts and regular use of air-conditioning.

Frieden said Florida has taken aggressive mosquito-control measures in the affected area, which should also limit further transmission. He said there is no cause for concern unless additional cases are identified after those measures had begun.

As of Thursday, the CDC reported 1,658 cases of the virus in the continental United States and Hawaii. None of those cases is a result of local mosquito transmission. Fifteen of those individuals were infected by sexual transmission, and there is one case of a laboratory-acquired infection. (The CDC's numbers do not always include the most current cases reported by states.)

Nearly every state is reporting cases of the virus. Only Idaho, South Dakota, Wyoming and Alaska have not reported it.

Risks for pregnant women

Pregnant women are at greatest risk because the virus can have devastating consequences for an unborn baby, including the birth defect microcephaly and other neurological deficits, as well as miscarriage and stillbirth among women who were infected while pregnant.

The exact risk that an infected woman will have an affected baby remains unknown, but some studies have shown that it is between 1% and 13%.

Frieden called this unprecedented , saying, "never before in history has there been a situation when a bite from a mosquito can result in such a devastating scenario."

Local transmission "is the news we've been dreading," said Dr. Edward McCabe, chief medical officer and senior vice president of the March of Dimes. "It's only a matter of time before babies are born with microcephaly, a severe brain defect, due to local transmission of Zika in the continental U.S. Our nation must accelerate education and prevention efforts to save babies from this terrible virus."

Frieden restated that it's important for pregnant women in any area where the Zika-carrying Aedes aegypti mosquito is found to protect herself against bites with the use of repellant and by wearing long pants and long-sleeve shirts.

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At least 13 infants have been born with Zika-related birth defects in the continental U.S. and Hawaii, and there have been six sudden or voluntary Zika-related pregnancy losses reported. There are more than 400 pregnant women with the virus in the United States.