NEW YORK -- Health officials in Minnesota reported a new case of the Zika virus Wednesday. This month, ten cases have turned up in five states.

The mosquito-borne infection is a threat to fetuses developing in the womb. All of the U.S. cases are in people who traveled south of the border. Now, health officials are worried about the likelihood of an outbreak here at home.

Brazil is fighting the spread of the virus by trying to control its mosquito population. So far, Zika has not been found in any mosquitoes in the U.S., but the types of mosquitoes that could potentially carry the virus are found in this country.

In fact, in warmer months, those mosquitoes can be found in regions where 60 percent of Americans live. That's about 200 million people.

The Zika virus has been linked to a birth defect called microcephaly, where babies are born with small heads.

Geovane Silva holds his son Gustavo Henrique, who has microcephaly, at the Oswaldo Cruz Hospital in Recife, Brazil, January 26, 2016. REUTERS

NYU Langone Medical Center's Dr. Tara Shirazian says doctors here are bracing for the likely arrival of the virus. About four million women in America get pregnant each year.

"Out of five women, only one exhibits symptoms," Shirazian told CBS News. "So the other four don't have symptoms and may still have been exposed and may still develop microencephaly in their fetuses. And the transmission from the mom to the fetus, we think, is relatively high."

Nurse practitioner Safiyyah Okoye is 22 weeks pregnant and has decided not to vacation in the Caribbean this year.

"Even some of the countries we were considering that weren't on the travel ban list, I felt like by the time of our trip in March, it seemed like the list was rapidly growing," Okoye said.

Both United and American Airlines issued new policies Wednesday allowing pregnant women to postpone travel or receive full refunds for flights to Zika-affected countries.

The common way Zika spreads is through mosquito bites, but more research is needed about the risk of transmission through sexual contact and blood transfusions.

A vaccine could be three to five years away.