(CNN) Pregnant women should not travel to Barbados, Bolivia, Ecuador, Guadeloupe, Saint Martin, Guyana, Cape Verde and Samoa because of Zika virus, the CDC said Friday.

This comes on the heels of last week's travel alert from the CDC recommending pregnant women postpone travel to Brazil, Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Suriname, Venezuela and Puerto Rico. The recommendations also call for women who have traveled to these places during their pregnancy be screened and monitored for the virus if their visit took place while the virus was present in the country they visited.

The decision is based on their ongoing monitoring of the Zika virus around the world and CDC say they will update as necessary in the interest of public health.

Zika virus is a mosquito borne disease. An individual becomes infected by the bite of an infected mosquito. Symptoms of the virus include fever, rash, joint pain and red eyes and can last from a few days to about a week. But 80% of individuals infected have no symptoms.

So why is there so much concern?

The virus has been linked to an increase in cases of a rare neurological condition called microcephaly in babies. Microcephaly results in babies being born with abnormally small heads, and often serious, and sometimes deadly, developmental delays.

This is what prompted the Salvadoran vice minister of health to tell women there not to get pregnant for two years. "We're recommending that women who may get pregnant plan their pregnancies and try to avoid getting pregnant this year and the next," Eduardo Espinoza told CNNEspanol Thursday.

Colombia and Jamaica issued similar recommendations for women to delay pregnancy earlier this week.

Brazil was the first nation to give this advice in November after they saw a sharp increase in the rare birth defect coinciding with an outbreak of the virus there. The Ministry of Health has recorded more than 3,700 cases of microcephaly in 2015 and so far this year. In comparison, 147 cases of microcephaly were recorded in 2014.

The CDC conducted tests on human tissue from babies in Brazil confirming a the link between Zika virus and microcephaly.

Health officials there and in El Salvador, also suspect a surge of the rare disorder Guillian Barre Syndrome , or GBS, is connected to Zika virus. GBS is a rare disorder in which an individual's immune system attacks nerve cells, resulting in muscle weakness and sometimes paralysis. The CDC is working with the Ministry of Health in Brazil and has a team on the ground conducting investigating a possible link between the two.

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"It's a strong message to tell people to stop getting pregnant for a couple of years," said Stephen Higgs, PhD, president of the American Society of Tropical Medicine and Hygiene . He likens it to the message to promote safe sex for the prevention of HIV. "Some people will pay attention to this and realize they are at risk for something, others are obviously going to ignore it," he said.

Higgs believes a unified message could help, noting that El Salvador recommends a two-year halt on pregnancy while Jamaica, for example is recommending 6 to 12 months. This could come in the form of an overarching announcement from the World Health Organization or the Pan American Health Organization that applies to all Zika-infected countries, rather than individual countries making recommendations. The message will probably change too as more is learned about the virus.

What now?

"Zika in the Americas is in new territory and (as) we've seen with the microcephaly, it's showing different patterns of infection that hasn't been seen before. We don't fully understand what's going on," Higgs said.

He also expects the CDC to continue to expand the list of nations as the virus continues to spread.

In the U.S. there are confirmed cases of the virus among individuals who have traveled to infected countries, including in Illinois, Florida and Texas, among others. However, there are no known cases of locally transmitted illness. "These imported cases might result in local human-to-mosquito-to-human spread of the virus in limited areas of the continental Unites States that have the appropriate mosquito vectors," according to a new report on the spread of the virus issued by the CDC Friday.

There is at least one known case of a baby born with microcephaly believed to be linked to Zika in Hawaii.

There is no prevention or treatment, which is why the best course of action is for travelers to danger areas is to prevent mosquito bites by using mosquito repellant and covering exposed skin. The aedis aegyptia mosquito, which transmits the disease, bites all day long, so individuals need to reapply that repellant and not let their guard down. Aedes albopictus mosquitoes, which are found throughout the U.S. and are known for transmitting dengue fever and chikungunya , may also transmit the virus, the CDC said Friday.

However, mosquito bites and mother to unborn baby aren't the only ways this virus is transmitted. The new CDC report notes documented cases of infection from sexual transmission, blood transfusion and laboratory exposure.

The good news is, while it's not certain, scientists believe once an individual has been infected with the virus, they are immune and won't become infected again, according to Higgs.