Zika first appeared in Africa and south-east Asia but without reports of microcephaly cases as experts consider whether to declare health emergency

The World Health Organisation’s director general, Margaret Chan, who took such flak over the mishandling of the early stages of the Ebola epidemic, is calling a meeting for Monday which could decide to declare the zika virus an international health emergency.

But though the WHO undoubtedly wants to move fast and decisively to show it has learned the lessons of Ebola, it is most unlikely to declare an emergency as yet.

Zika is spreading “explosively” in the Americas, Chan told member states in a briefing on Thursday. But, the WHO’s assistant director general, Bruce Aylward, said within an hour or so that they were concerned rather than alarmed.

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There is real concern over the birth defects known as microcephaly in areas that have had zika cases, but no one can yet say categorically that zika is the cause.

Zika is spreading like wildfire across the Americas because the population has no immunity to the virus. This is a new virus, spread by a species of mosquito that is very common in the region and which already transmits dengue fever, caused by a virus from the same family. Two more that are related are yellow fever and chikungunya virus.

Because the symptoms of zika can be very mild, most people infected are unlikely to have gone to a doctor, but modelling the spread of zika from what is known of dengue fever, which infected 2 million humans last year, epidemiologists say we can expect 3 to 4 million cases a year.

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Zika has been linked to thousands of birth defects in South America but the virus was in other parts of the world before it got there. It appeared in Africa, south-east Asia and the Pacific islands prior to 2015, but without reports of microcephaly too. Experts are now urgently investigating to see if incidents were missed.

If zika is implicated in South America it is possible that it is not the sole cause. Further investigations will look at whether women whose babies were born with defects had been infected by dengue or chikungunya as well, in case a combination of viruses is to blame. They will also want to find out how many babies of women with zika virus have been born without the birth defect.

When Ebola spread across west Africa and the WHO for too long failed to call an international health emergency, there was no doubt the virus was killing people in a terrible way and that the infection was being passed from one person to another. Zika is a massive worry, but investigations and warnings to women who are or might become pregnant are likely to be the main response for now.

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Chan is calling the meeting for Monday in part because she is afraid that people and businesses will boycott Brazil and other affected countries. That was one of the reasons for delaying the declaration of an emergency in Sierra Leone, Liberia and Guinea, all of which were badly hit when airlines stopped flying and businesses shut up shop.

So for now, a declaration of international health emergency in South America is unlikely. But the answers to a lot of questions need to be found fast.