While there are many cases of Zika at present spread across more than 30 countries, it has proven nowhere near as deadly. In Brazil, the country hardest hit by the outbreak so far, the government thinks that as many as 1.5 million people may have been infected with the virus so far. Yet there's only been a handful of adult deaths linked to the virus. In fact, experts believe that only one in five people infected with Zika experience any sort of symptoms at all.

What's actually concerning about Zika isn't what we know, however. It's what we don't know. These "known unknowns" reveal just how little we really understand about this Zika outbreak so far. And while the international community has moved quickly into action – quicker than it reacted to Ebola, experts say – the scale of what we don't know is still a huge cause for concern.

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“This emergency is because of what’s unknown,” David Heymann, an infectious-disease professor at the London School of Hygiene and Tropical Medicine told The Washington Post's Lena Sun and Brady Dennis recently. “The Ebola emergency was because of what was known.”

It's a complicated and shifting picture. Here is some of what we still don't know.

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Whether there is a link between Zika and microcephaly

One reason this latest Zika outbreak is causing so much concern is because its arrival in Brazil appears to have coincided with a surge in the number of cases of microcephaly, a rare neurological condition which leaves children with unusually small heads. In many cases of microcephaly, the child's brain may not develop properly. In Brazil, there has been an abundance of more severe cases, including a number of cases where the child died shortly after birth.

Brazilian doctors say they have seen around 4,000 cases of suspected microcephaly since last fall, a huge surge considering that they reported 147 cases total in 2014. Many suspect that there is a link between the Zika outbreak and the rise in cases of microcephaly. Albert Ko, an epidemiologist and infectious disease expert at the Yale University School of Public Health who spent time in the Brazilian city of Salvador last December, recently told The Washington Post: "We strongly believe that the cases of microcephaly we are identifying during this outbreak are due to Zika virus."

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The evidence for a link between Zika and microcephaly has become more and more compelling. Evidence of the virus has been found in amniotic fluid of pregnant women carrying fetuses with microcephaly, in the brain tissue of infants with microcephaly who died shortly after birth, and in the placenta of women who have miscarried fetuses with the condition.

There are still some wrinkles, however. Some studies say that the rise in the microcephaly cases in Brazil may actually predate the Zika outbreak. And as The Post reported late last month, there have been some signs that there may actually have been fewer microcephaly cases in Brazil than first suspected.

Whether there is a link between Zika and other health issues, including one rare syndrome that can lead to paralysis

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Another big worry for health officials is whether there is a link between the Zika outbreak and a growing number of cases of Guillain-Barré Syndrome (GBS) – an immune syndrome that can lead to paralysis, and, in rare cases, even death. In El Salvador, for example, there appears to have been a surge in the numbers of cases of GBS: In a little over a month there were 46 cases recorded, while usually there is an average of 169 a year.

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Notably, while a surge in the cases of microcephaly appears to be limited to Brazil so far, a large number of countries have reported a rise in the number of GBS cases. According to a recent WHO report, French Polynesia, Brazil, El Salvador, the French territory of Martinique, Colombia, Suriname, Venezuela and Honduras have all seen an increase in GBS cases.

This isn't the only other issue that may be linked to Zika. Doctors have observed a large number of major eye defects in babies born with microcephaly. Speaking to The Post, epidemiologist Ko suggested that he was seeing a whole "spectrum" of birth defects. "It seems like microcephaly may just be the tip of the iceberg,' he said.

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How Zika can be transmitted

We know that the virus is spread primarily through bites from the aedes aegypti mosquito, though the aedes albopictus mosquito is also capable of spreading the virus (this is of particular concern in the United States, where the aedes albopictus has a far larger geographic footprint).

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But mosquitos may not be the only culprit. The Centers for Disease Control and Prevention recently confirmed that sexual transmission of the Zika virus was "possible" and Dallas health officials have said that one local resident was infected with Zika after having sex with a person who had contracted the disease while traveling.

Exactly how this works is unclear. Recent CDC guidelines note that in the cases where the disease was sexually transmitted, the men had developed the symptomatic illness. It is unknown, the CDC guidelines say, whether "infected men who never develop symptoms can transmit Zika virus to their sex partner." There have been no reports of infected women transferring the virus to their partners. The CDC is taking the issue of sexual transmission seriously enough to tell men who have traveled to countries with a Zika outbreak that they should consider abstaining from any sex if their partner is pregnant.

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This isn't all. Officials in Brazil have said that the disease has been spread via blood transfusions, something which the CDC and other health bodies had previously warned was possible: The Food and Drug Administration recently advised anyone who could potentially have been exposed to Zika to avoid donating blood for at least four weeks. And scientists have discovered that the Zika virus is "active" in saliva and urine. While it remains unclear if it could actually spread via either of these, Brazilian health officials have warned pregnant women to stay away from crowds and avoid any chance of sharing saliva with an infected person, be it by kissing or sharing cutlery.

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The real extent of Zika

The fact that just one in five people infected with Zika show any symptoms may seem like a good thing. In reality, however, it also makes it remarkably difficult to know how many people are actually infected with Zika at any one time. Without symptoms, it becomes vastly impractical to test for Zika – it's especially difficult in pregnant women, as the virus only stays in the blood for around a week. After that, it becomes difficult to distinguish from dengue and chikungunya, two related-yet-separate diseases.

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Past outbreaks have had a high infection rate. During a 2007 outbreak on the Yap Island in the Pacific, an estimated 73 percent of people over the age of two were infected.

This means that in some parts of Latin America, pregnant women are just sitting and waiting. Only Brazil has reported a surge in cases of microcephaly so far, but other countries may well follow suit. Experts have pointed to Colombia, where mosquitos carrying the pathogen are believed to have arrived around five months after they reached Brazil, and warned there could soon be a "time bomb" of microcephaly cases. Some countries, such as Venezuela, are thought to be seriously lowballing the toll Zika is already taking.

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Last month, WHO warned that the virus was spreading so fast that it could infect as many as 3 million to 4 million within the next 12 months. The virus is already in scores of countries and it is likely to spread around the world: Experts have warned that the United States may be vulnerable.

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How to end this outbreak

This latest outbreak has led to some radical-sounding plans for how to eradicate Zika: Either producing genetically altered mosquitoes or perhaps simply killing mosquitos all together. One state in Brazil has banned the use of pesticide Pyriproxyfen, a larvicide used to destroy mosquito eggs and larvae, following suggestions it was linked to the Zika outbreak – despite various health authorities saying there was no scientific basis behind the ban.

One of the simplest ways to contain the virus – a vaccine – remains out of reach. Creating vaccines is often a difficult, expensive process that can take large amounts of time and there had been little interest in a Zika vaccine prior to this outbreak. The World Health Organization recently stated that possible Zika vaccines were at least 17 months away from large-scale trials, despite the large amount of resources being thrown behind their development. There are some hopes that a vaccine could be found sooner: Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, recently told The Post that due to previous research done on two similar viruses (West Nile and dengue), it may be possible to accelerate the creation of a vaccine against Zika.

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For now, officials are left clutching at straws. A number of Latin American countries have asked women to avoid becoming pregnant for the time being: El Salvador has suggested women should hold off on pregnancy until 2018.

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These blank spots are a legitimate cause for alarm. They also have the knock-on effect of creating a fertile Zika conspiracy theory cottage industry, where theories that link the outbreak to genetically modified-mosquitos seem to be most popular. Such theories are easily debunked, but they spread as they help people make sense of the virus.

But why are there so many blank spots here? Zika isn't a new virus. It was first discovered in 1947 (it's named after the Zika forest of Uganda, where it was discovered).

The thing is, for decades Zika wasn't really seen as a threat. Few had even heard of it. And, as CDC Director Tom Frieden recently pointed out on Twitter, the amount of literature about it is remarkably limited. Attempts to get funding to study Zika failed because, as one scientist put it, "Zika isn't important."

Zika's sudden move from an obscure, "not important" virus to one of the globe's most important health emergencies took most of the world's scientific community by surprise and the known unknowns are what makes the virus so concerning. The virus has proven to be more dangerous than anyone thought. And, more worrying still, the unknown unknowns have the capacity to be worse.