(CN) - Scientists warn that more than 2.5 billion people - 35 percent of the world's population - are at risk of being infected by the Zika virus.
The World Health Organization said the outbreak of the virus remains an international health emergency, noting Zika's spread to new nations including the United States, Singapore and Malaysia.
In a study published this past week, researchers determined after analyzing travel, climate and mosquito patterns in parts of Asia and Africa, that more than 2.5 billion people could contract Zika, concluding that people in these regions are particularly at risk.
The researchers also pointed out that areas vulnerable to outbreaks include some of the world's most populous nations, many with limited resources to identify and stem the spread of the mosquito-borne virus. Their findings were published in the journal The Lancet Infectious Diseases.
After meeting this week to assess the current status of the epidemic, the United Nations health agency determined that considerable gaps remain in understanding Zika and its connection to severe health conditions.
With many questions still unanswered, the WHO decided the outbreak remains a global emergency.
Guillain-Barre syndrome - an autoimmune disorder that causes the body to attack parts of the peripheral nervous system - has been thought to be caused by Zika infection in a small number of adults.
Further evidence of Guillain-Barre's connection to Zika was presented in a report published Wednesday, which found a strong connection between the disease and the virus.
The report - a letter to the editor of the New England Journal of Medicine - found that nations experiencing an increase in Zika diagnoses have had an identical spike in Guillain-Barre cases.
"It's pretty obvious that there is a clear relationship," lead author Marcos Espinal, director of communicable diseases at the Pan American Health Organization, wrote.
The most vulnerable nations include China, the Philippines, Indonesia, Nigeria, Vietnam, Pakistan, India and Bangladesh, according to the research.
While the study estimates that more than 2.5 billion people are at risk of contracting Zika, experts noted that the figure could be an overestimate because the virus may have already spread in some of these nations in the past and immunity may have already developed.
The WHO also said that Brazil has not reported any confirmed cases of Zika following the Rio de Janeiro Olympics.
Zika has infected 72 nations and territories.