The Zika virus, a mosquito-borne disease suspected of causing serious birth defects, is expected to spread to all countries in the Americas except Canada and Chile, the World Health Organization said. Zika has suspected but unproven links to microcephaly — in which babies born to women infected during pregnancy have abnormally small heads. The virus is already present in 21 of the 55 countries and territories across the Americas, the WHO said in a statement Sunday.

But it stressed that the Aedes aegypti mosquito, which carries Zika and also dengue and chikungunya viruses, is already present in all countries in the Americas besides Canada and Chile. WHO pointed out that since people in the region had not been exposed to Zika before it emerged in Brazil last May they lacked immunity, allowing the virus to spread quickly. FULL REPORT