The United States Center for Disease Control has issued a travel advisory for pregnant women to avoid neighborhoods in Miami, Florida, warning against risk of Zika.

News of the travel advisory comes after four cases of Zika virus were discovered July 29 and health officials believed that infected mosquitoes were transmitting the virus within the U.S.

Read More: Zika Is Likely Being Transmitted by Mosquitoes in the US

The CDC issuing a travel advisory within U.S. continental borders is another first. Previously, the CDC has only issued travel warnings abroad.

After the Zika count rose from four to 14 people over the weekend, federal health officials decided to issue the warning for the Wynwood arts district — a Miami neighborhood where all 14 cases were found.

Health officials believe mosquitoes were infected within the U.S., meaning that uninfected mosquitoes bit a person who already had the virus.

Travel agencies, like Jet Blue, are even required to honor travel refund to Miami.

However, Gov. Rick Scott said, “While I encourage all residents and visitors to continue to use precaution by draining standing water and wearing bug spray, Florida remains safe and open for business.”

Other agencies, like Public Health England, put the risk for contracting the Zika virus in Miami at moderate, alongside other tropical zones such as Guinea-Bissau, Indonesia, and the Philippines. Higher risk countries include those in the Caribbean, and parts of Latin America, like Brazil.

Brazil, host of the 2016 Summer Olympics beginning in just days, has seen the most damage from the virus.

1,600 cases of microcephaly (a serious birth defect inhibiting brain development in fetal stages and by the Zika virus) appeared in Brazil connected to Zika, and another 3,000 are still under investigation by Brazilian health officials.

Read More: Zika virus causes concern over the 2016 Olympics

Concern over Zika in Brazil has caused athletes to make the tough decision not to participate in the Olympic games. Others plan to take precautions such as wearing “Zika-proof” uniforms, or freezing sperm in case of infection as the virus is sexually transmittable according to the Atlantic.

Now, more travellers will have tough decisions to weigh on upcoming travels while health organizations do their best to mitigate the spread of Zika.