This 2006 photo provided by the Centers for Disease Control and Prevention shows a female Aedes aegypti mosquito in the process of acquiring a blood meal from a human host.

The CDC's interim travel guidance is for 14 countries and territories in Central and South America, as well as the Caribbean, where Zika virus transmission is ongoing: Brazil, Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Suriname, Venezuela and the Commonwealth of Puerto Rico. Those who go ahead and travel anyway have been cautioned to take care to avoid mosquito bites, which are a risk both indoors and outdoors, mostly during the daytime.

The CDC further advises that pregnant women who visited countries where Zika is spreading should be tested for the disease if they have two or more symptoms, such as joint pain, fever and rash.

Deputy medical health officer for the Saskatoon Health Region Dr. Johnmark Opondo, said they are most concerned about microcephaly, a developmental disorder where newborns are born with abnormally small heads.

Colombia's Deputy Health Minister Fernando Ruiz also cited that 12 cases have been reported of people with a neurological syndrome that resembled symptoms of the Zika virus and corresponding with Guillain-Barre syndrome, which is a condition in which the immune system attacks the nerves. The virus is not spread from person to person.

Researchers in Brazil are working on a vaccine to prevent Zika but there is no treatment available. The surge of cases since the new virus was first detected a year ago in Brazil led the ministry to link it to the fetal deformations and warn pregnant women to use insect repellent to avoid mosquito bites.

The guideline announcement came as two pregnant women in Illinois were reported on Tuesday to have contracted the disease, according to the Illinois Department of Public Health.

New cases of Zika virus infection have been reported from Florida and IL.

"That's a concern for us here because we are a major tourist destination for visitors from the epidemic areas", said Dr. Antonio Crespo, an infectious disease specialist at Orlando Health. The announcement was made by the southern state of Parana branch of Fiocruz research institute.

The virus is also moving to new areas when the mosquitoes themselves travel to different countries, either by hitching a ride on travelers or cargo, Osterholm explained to Bloomberg.