The Zika virus was discovered in a monkey in Uganda in 1947, and the first human case was registered in Nigeria in 1954. Borne by mosquitoes, it has circulated in Africa and Asia for decades, and more recently in the Pacific region, never causing much concern.
The situation today is "dramatically different," World Health Organization (WHO) chief Margaret Chan said on January 28.
The virus "is now spreading explosively" after being detected in Brazil last year, Chan said. She said that "the level of alarm is extremely high."
Chan called for an emergency meeting on February 1 to seek "advice on the appropriate level of international concern and for recommended measures that should be undertaken in affected countries and elsewhere."
Zika virus infections are now occurring in some 20 countries and territories in the Caribbean and North and South America -- more than double the number in the region a month ago.
Zika infection is usually asymptomatic -- meaning that the infected person has no symptoms -- or causes mild illness that normally lasts for up to a week, with symptoms such as fever, rash, joint pain, and conjunctivitis. Severe disease and fatalities are uncommon.
But Lawrence Gostin of the O'Neill Institute for National and Global Health Law in Washington tells RFE/RL that the deepest concern is that the disease, which he says has "explosive" pandemic potential, is now suspected to result in fetal abnormalities.
WHO chief Chan highlighted the growing concern that Zika has links to a birth defect known as microcephaly, in which infants are born with abnormally small heads.
"A causal relationship between Zika virus infection and birth malformations and neurological syndromes has not yet been established, but is strongly suspected," she said on January 28.
The Aedes species mosquito that transmits the Zika virus -- as well as dengue, chikungunya, and yellow fever -- occurs worldwide, posing a high risk for global transmission to geographical areas with little population immunity.
"I can foresee within the next year or two there being very substantial outbreaks in North America, Europe, and other parts of the world," Gostin says. "It's quite serious and we need to take it very seriously."
Infographic: Areas With Active Zika Virus Transmission:
Brazil, which hosts the Summer Olympic Games in Rio de Janeiro in August, has been hit hardest by the outbreak, with more than 1 million cases reported.
Brazil has also recorded a dramatic spike in cases of microcephaly, which can cause severe developmental issues and sometimes death.
Nearly 3,900 cases have been reported in the country of 200 million since October, compared with the previous annual average of just 160 cases. And evidence of the Zika virus has been found in the brains of fetuses or newborns and in the placenta and amniotic fluid of mothers.
No Treatment, No Vaccine
There is no specific treatment or vaccine for Zika, and no quick diagnostic tests available.
Scientists struggled for years to develop a vaccine for dengue, which is related to the Zika virus; the first such shot made by pharmaceutical company Sanofi Pasteur was licensed last year in Brazil.
For people in affected areas, WHO recommends clearing stagnant water where mosquitoes breed. It also calls on people to protect themselves from mosquito bites by using insect repellent, wearing clothes that cover as much of the body as possible, and closing doors and windows.
Citizens and governments in Latin America are struggling to fathom the threat.
"We are all afraid of [mosquitoes], it makes our daily life very complicated," Bruna, a resident of Sao Paulo, Brazil, told Reuters. "The Zika virus spreads very quickly, which makes all of us very surprised. We all avoid going to forest areas when traveling."
The price of mosquito repellent has reportedly risen in Brazil.
In the northeastern city of Recife, reports said public hospitals were packed with pregnant women who fear their babies may have an underdeveloped brain. Others wanted their newborns to have their heads measured.
Officials in Rio this week dispatched a team of fumigators to the Sambadrome, where the city's Carnival parades will take place in February, and the regional governor was distributing mosquito-fighting vehicles for poor suburbs of the city.
Meanwhile, Brazil's government announced that 220,000 soldiers would be deployed to hand out leaflets on how to fight the further spread of the virus.
Other countries made more drastic moves. Health officials in El Salvador have advised women not to get pregnant until 2018, and authorities in Colombia, Ecuador, and Jamaica also urged people to wait -- though not quite that long.
"If the country is going through a very substantial outbreak like Brazil, I think advising women to delay their pregnancies for a short time until more is known is probably wise," Gostin says. "I think it's an overreaction what [El Salvador] did; you don't want to skip a generation."
The United States, Britain, and other countries have recommended that women who are pregnant or planning to become pregnant postpone travel to affected areas.
Zika has been detected in the United States and a growing number of European countries in people who had recently traveled to Latin America. However, there have been no known cases of local transmission in the United States or Europe so far.
Scientists say the virus may have reached Brazil with tourists who streamed into the country for the 2014 World Cup soccer tournament. Another possibility is that participants in a canoe race in Rio brought it over from French Polynesia.
Accelerated research is under way to develop vaccines and treatments and to better understand the virus and how it affects babies, but Gostin says it could be a decade before a vaccine is publicly available.
Time may be running out.
The WHO expects the Zika virus to reach parts of every country in the Americas except for Canada and Chile, which aren't home to the type of mosquitoes that spread it.
U.S. National Institutes of Health Director Francis Collins cited a study earlier this month in which researchers warned the virus may affect regions in which 60 percent of the country's population lives.
With reporting by AFP, Reuters, AP, and The New York Times