The U.S. Centers for Disease Control and Prevention has added four more countries to the growing list of places where travelers are at risk of getting infected with the Zika virus.

On Monday, the CDC added American Samoa, Costa Rica, Curacao, and Nicaragua to the list that is already containing 28 other regions, Reuters listed (via AOL.com).

The CDC issued its first travel alert in January and has since been updating the list, which cautions pregnant women in particular, Reuters added.

The World Health Organization, or WHO, declared on Monday that the Zika outbreak is a "public health emergency of international concern," The Telegraph reported. That presented repercussions in Brazil, where the 2016 Olympic Games will be held on August 5 in Rio de Janeiro.

"The risk, which I would say is serious, is for pregnant women. It is clearly not advisable for you (to travel to the Games) because you don't want to take that risk," said President Dilma Rousseff's chief of staff, Jaques Wagner, as quoted by the news outlet.

Zika is connected with microcephaly, a rare birth defect in which infants are born with shrunken skulls and brains that do not develop properly. Brazilian researchers confirmed that the virus can be transferred from the mother's placenta to the fetus, according to a report from Wired.

Some health officials also said that Zika is the root of Guillain-Barre syndrome, a rare condition in which the immune system attacks the nervous system, causing weakness and paralysis at times, The Guardian further reported. The disorder directly affects patients infected with Zika. Majority of them recover, but the condition is sometimes fatal.

Brazil was the first country to suspect of Zika's link to microcephaly, the news outlet noted. It has been declared as the worst affected nation, where 270 cases of microcephaly have been confirmed since October 2015, while around 3,500 suspected cases are still being investigated. The governments of Colombia, Ecuador, El Salvador, Jamaica, and Puerto Rico have also advised women against pregnancy.

The WHO said that the virus, which is transmitted by the Aedes aegypti mosquito, is "spreading explosively" in the Latin American region, predicting that up to four million Zika cases will occur this year alone, The Guardian wrote.

There is currently no specific treatment for the Zika virus, The Guardian added. The WHO asserted the need to advance diagnostics and develop a vaccine for the disease.

Wagner said that it will take researchers between three to five years to develop a vaccine against Zika, Stuff.co.nz reported. The chief of staff added that Brazil's researchers are working alongside experts in the United States.