Global health authorities are warning that the Zika virus is predicted to spread through South, Central and North America and will likely reach all countries and territories where the Aedes mosquitoes are found.

Since Brazil reported the first case of local transmission last year, the mosquito-borne disease has spread to 21 countries and territories of the Americas, the World Health Organization said in a statement. At least a dozen cases have been confirmed in the United States by the Centers for Disease Control and Prevention.

WHO’s regional office for the Americas said there are two main reasons for the “rapid spread.”

“The population of the Americas had not previously been exposed to Zika and therefore lacks immunity,” the U.N. health agency said in the statement. The WHO added that “Aedes mosquitoes – the main vector for Zika transmission – are present in all the region’s countries except Canada and continental Chile.”

The Zika virus has ravaged Brazil, which has had more than 1 million cases in the past several months. It is transmitted through the Aedes mosquito and usually causes a mild fever and a skin rash, along with conjunctivitis (or pink eye) and muscle or joint pain, according to the WHO.

However, some babies exposed to the virus have suffered serious brain damage – prompting authorities to act.

Many pregnant women in Brazil now fear there may be a link between the virus and a rare birth condition called microcephaly, which is associated with incomplete brain development.

In addition, many also worry about infection from dengue and chikungunya viruses, which are carried by the same mosquito.