What we don't know is often scarier than what we do, especially when it comes to public health, and earlier this week the World Health Organization declared an international public health emergency over the Zika virus, which in its words is 'spreading explosively across Central and South America.'

Now if there are three words you never want to hear in the same sentence, they're probably 'virus', 'emergency' and 'explosively'. But there's a lot we still don't know about Zika and scientists are in a hurry to learn as much as they can, because while this virus has been thought for decades to be practically harmless, it may turn out to be responsible for a rash of serious birth defects.

(0:45) So here's what we know so far:
Zika is a virus, named after the Zika Forest in Uganda where it was discovered in 1947. It's from the family of viruses known as flavivirus, which also includes the dengue, yellow fever and West Nile virus, none of which are at all pleasant. 

But infection with Zika seems most often to create little more than mild flu-like symptoms, if any at all. It's spread by a certain kind of mosquito known as aedes- tropical blood suckers that spread other flaviviruses, and for decades, Zika has been circulating around parts of Africa and Asia, but it didn't appear to be doing much damage. People who were infected would come down with a mild fever, a rash, maybe a headache, but that was it. Up to 80% of people showed no symptoms at all.

So it's been thought that a stable relationship was struck between human, mosquito and virus. People who were infected at an early age suffered mild illness and then developed immunity before they were old enough to get pregnant. 

(1:35) Then, all of a sudden in 2007, the virus moved. There was a Zika outbreak on Yap- an isolated set of islands in the south-western Pacific. Nearly three quarters of the population were infected, but again, the results were mild. Nobody died or was even hospitalized.

Then, in November 2015, health officials in Brazil raised alarm about a worrying trend. Last year, nearly four thousand babies were born with extremely small heads; a condition known as microcephaly. For each of the previous five years, this number was more like 150, and this was bad news. If babies do survive being born with microcephaly, they can suffer severe physical and learning difficulties their whole lives.

Now, many things can cause microcephaly including genetics and malnutrition and heavy alcohol use during pregnancy, but none of these things could explain Brazil's sudden increase in birth defects. 

What was new to Brazil that year was Zika, first detected there in May 2015, but largely ignored. There was no reason to suspect it, but tests on a handful of pregnant women with affected babies revealed the presence of RNA from the Zika virus.

Now, it's not certain that Zika and microcephaly are related, but new evidence is coming in almost daily and as of now, officials are assuming that they are related.

(2:43) Unfortunately, there's no vaccine for Zika and it could take a decade to develop one. As a result, advice from the Centers for Disease Control and Prevention is based entirely on avoidance and vigilance. For example, women who are pregnant or think they may be should avoid any of the twenty-six countries where Zika is active, mainly in Central and South America, and those already living there should do whatever possible to avoid mosquito bites, like using repellent, wearing long sleeves and trousers, and removing standing water where the mosquitoes breed.

In the meantime, researchers are working to clear up many things about Zika that we don't know, like was Zika causing microcephaly all along in Africa and the South Pacific, and it just wasn't reported? Or is there just something different about the virus in Brazil or the way it interacts with people there?

While doctors tackle those questions, other scientists are focused on stopping the spread of the virus.

The aedes mosquito lives mainly in tropical and sub-tropical regions and it's not likely that it will ever leave that range. The main worry is that the virus will find a different way to get around, or mutate into an even more dangerous form like the West Nile virus did back in the nineties. 

Viruses are master mutators and humans are master travelers, both of which could help Zika spread. Some experts think that Zika came to Brazil around the time of the 2014 World Cup among the thousands of traveling sports fans, and this summer, Rio is hosting the Olympic Games, so it seems like we have a lot to learn in very little time.

(3:58) Thanks for watching this episode of SciShow news, and thank you especially to all of our patrons on Patreon who make this show possibly. If you want to help us keep making videos like this, you can go to patreon.com/scishow, and don't forget to go to youtube.com/scishow and subscribe.