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.”

Copyright (c) 2015. RFE/RL, Inc. Reprinted with the permission of Radio Free Europe/Radio Liberty, 1201 Connecticut Ave NW, Ste 400, Washington DC 20036.