What is the Zika virus?

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Young residents ran away and others covered their faces as a municipal worker sprayed insecticide to combat the mosquitoes that transmit the Zika virus.Felipe Dana

In a matter of months, the mosquito-borne Zika virus has gone from little-known nuisance to South American scourge and, increasingly, global public health crisis.

The World Health Organization announced Thursday that it is putting together an emergency committee to deal with the fast-spreading virus, and the US Centers for Disease Control have recommended that travellers to Zika-infested countries take "enhanced precautions," with pregnant women urged to consider alternate destinations.

Never before has there been a Zika outbreak of this scale, with over a million cases in Brazil and potentially millions more in the months ahead.

Especially worrisome is the fact that the disease seems to be causing a rare birth defect called microcephaly — where babies are born with underdeveloped brains — and possibly also the paralysis-causing nervous system disease Guillain-Barre.

Why is Zika suddenly spreading?

The Zika virus isn't new. It was first identified over half a century ago, and has long circulated in Africa and south-east Asia. But for decades, it has had a reputation as a relatively mild illness, not the source of pandemics.

In 2013, though, there was a surprisingly widespread outbreak in French Polynesia, along with the first hint that the disease might be causing microcephaly.

A similar version of the virus surfaced in Brazil last year, rapidly making its way across the country — and, increasingly, the entire continent. Again, most people had mild symptoms, but incidents of microcephaly have risen markedly, from under 200 to over 4,000 in just one year.

As to why Zika has suddenly started to spread more aggressively, there are a couple of possibilities.

It could be a genetic mutation, some change in the virus which has enhanced its ability to infect adults and harm unborn children. Pinning this down would require some detailed genetic work, and because Zika has long seemed a relatively mild disease, there's little existing research.

Alternatively, it may be less about the virus and more about its victims. In Africa and south-east Asia, long exposure to Zika may have given people some immunity, which functioned as a kind of check on the virus's spread. But in the Western Hemisphere, no one has immunity, leaving Zika free to do its virulent work. In some ways, this explanation would count as good news, suggesting that today's continent-covering sickness may give way to widespread immunity within just a few years.

What can be done to stop Zika?

Zika is a mosquito-borne virus, in the same family as yellow fever, dengue, and west nile. So one way to stop the disease is simply to stop the mosquitoes. That means treating the water where mosquitos breed, and encouraging people to take basic precautions, like wearing insect repellent

This may seem like a second-best kind of response — compared to medical intervention— but for now there is no good medical treatment, and little hope for a quick fix.

Plus, mosquito control efforts can be effective. It's worth remembering that the CDC started life as an anti-malaria agency focused largely on mosquito control policies. For a long time, mosquito removal was a major part of US public health, and it seems to be a growing part of the response in Brazil.

Beyond this, however, infected countries are struggling to mount a meaningful response. Several have recommended that women put off getting pregnant for months, if not years, because of the risks of microcephaly. But as a social policy, discouraging pregnancy seems at best a very short-term solution. And even if it did work, it would have some pretty perverse side effects. Just think of the economic and logistical nightmare for pediatricians, obstetricians, day care providers, and schools with missing grades.

What about a vaccine?

Vaccines take time, and success is uncertain. Most vaccines operate on a simple principle: When you give someone either a weakened (or dead) form of the virus, their immune system often learns how to defend itself against the real thing.

But this approach doesn't always work.

One problem is that some diseases can strike twice, or even more. When you get dengue, for instance, your body doesn't develop immunity, so you can actually get it again. That is one reason people have been failing to make an effective dengue vaccine for over eighty years.

Zika is in the same family as dengue, which could create similar difficulties. Then again, it's also related to yellow fever, which you can stop with a fairly simple, and remarkably effective vaccine developed in the 1930s.

First Ebola, now Zika, what’s the lesson?

One thing this year's Zika scare shares with the 2014 Ebola outbreak is that, in both cases, a relatively confined disease suddenly surged into a pandemic.

Never before had Ebola killed more than a few hundred, until it hit cities in western Africa and took over 11,000 lives. Now with Zika, a virus long considered weak and limited has been transformed into a far-reaching force, poisonous for pregnant women.

But how to stop these diseases that explode into virulence?

In the wake of the Ebola experience came a call for an international fund to support vaccine development, with money from governments, foundations, and other sources.

The idea isn't to create millions of vaccines for dozens of different diseases, and keep them in a kind of stockpile. It's simpler, and far cheaper.

With enough seed money, researchers might be able to start down the path of vaccine-creation, for lots of different viruses — with trials on animals and preliminary safety assesments in humans. That way, when unexpected outbreaks did occur, the materials would already be in place for widescale production and broad human testing.

Such an approach might dramatically speed the process of getting urgent vaccines to doctors and patients. But, to be fair, even this might not have helped with Zika. When the New England Journal of Medicine published this proposal for an international vaccine fund, it also printed a list of diseases ripe for vaccine research. Zika wasn't on it.

Will I get Zika?

Several US citizens have already been struck with Zika, and one infected woman in Hawaii recently gave birth to a baby with microcephaly.

But none of these people got Zika from mosquitos in the US. Most of them contracted the virus while visiting Zika-infested parts of the world. Another developed symptoms after having sex with an infected person, which is a rare but occasional avenue of transmission.

Eventually, though, it's possible that Zika will find a home in the US. When a mosquito bites an infected person—perhaps just returned from a trip to Brazil—that mosquito becomes a carrier, passing the virus to future bite-victims and setting off a chain reaction where a growing number of sick people pass the virus to hungry mosquitoes who then fly off and infect others.

Still, there's no guarantee this will happen. Dengue, for instance, could spread this same way, but it doesn't—possibly because the US has become a largely indoor society, with air conditioning keeping us from exposure to mosquitoes, limits their ability to spread infections.

Massachusetts is especially unlikely to become a haven for Zika, because we just don't have the right kinds of mosquitos. So even if a few Bay Staters get sick from their travels, there's no easy way for the virus to spread.

The real risk seems to lie in the southeastern US, where Zika-friendly mosquitoes are more plentiful.

Yet it may be months before we really know how far Zika can spread. Right now, it's winter, but mosquito season isn't all that far away.