People are still talking about the Ebola virus and its deadly outbreak in West Africa, and now a new virus is making the headlines: mostly innocuous and fairly unknown until a few weeks ago, the Zika virus is suddenly dominating the news for its putative link with a congenital birth defect that causes babies to be born with abnormally small heads and undeveloped brains.
But what is the Zika virus, and how can it be harmless to most people yet cause such harm to an unborn fetus? To answer this question we have to take a step back and understand how viruses work and why some are endemic in the population, while others seem to come and go in waves.
The Zika virus was first isolated in 1947 from a rhesus monkey and from a pool of mosquitos from the Zika forest, in Uganda. It belongs to the same family of viruses as dengue, yellow fever, and West Nile virus. However, unlike its close relatives, Zika was thought to be relatively harmless: most infected people will experience no symptoms and a few just a rash and mild fever. Originally confined to Africa, it started spreading to Asia in 2007. Since then the spread of the virus has been exponential.
Viruses like Zika and Ebola replicate in animal reservoirs, i.e. populations where they are endemic. Ebola, for example, is usually found in bats and jumps to humans through consumption of meat from infected animals. Zika is found in monkeys and both monkeys and humans contract it through bites from mosquito carriers. In order to evade the host’s immune system, viruses evolve continuously: as organisms build immunity to fight them off, viruses accumulate genetic changes that enable them to escape the newly made defenses.
Contrary to Ebola, Zika is a less spectacular virus in many ways. It’s much smaller, and most of the people who contract it don’t even realize they’ve been infected except for a pesky mosquito bite. But that pesky mosquito bite is exactly the virus’s added strength: it becomes an invisible enemy, one that hides and migrates through a tiny insect. You can stay away from infected people when you see them sniffing and sneezing, but how do you avoid a symptomless agent that spreads through a flying bug?
You don’t. In areas where these mosquitos are endemic, children get infected early in life, build immunity against the virus, and don’t worry about it ever again.
Why is Zika posing a threat now, then?
The problem arises when the virus moves to a new geographical area and encounters a population that has never been infected before. Pregnant women are particularly at risk: unless they’ve been infected earlier in life, in which case their immune system can clear the infection before it reaches the fetus, any disease agent that has the ability to cross the placenta is a potential threat. Even a virus with normally mild symptoms like Zika, when it reaches the completely naïve immune system of a fetus in the early stages of pregnancy, can potentially cause permanent damage.
Currently, the connection between microcephaly and Zika is still putative and has yet to be confirmed. The danger, however, is real. As Los Alamos National Laboratory scientist Brian Foley explains, over the last two decades, vector-borne viruses like Zika and yellow fever have spread globally at increased rate. That human behavior is once again responsible for this new spread comes as no surprise. Increased traveling between continents, an exponentially growing population and, least but not last, a rise in temperatures have created the perfect haven for mosquitos—and hence the diseases they may carry—to spread virtually unstopped. Densely populated areas that are humid and riddled with stagnant water become the ideal habitat for these bugs.
The race for a vaccine has started, and several companies have already announced a time schedule to begin human trials in the near future. Zika is not a very diverse virus like HIV, for which the making of a vaccine has turned out much more challenging than originally anticipated. However, making any vaccine is regulated by strict government safety rules that require years of testing. “Under normal circumstances, it takes 10-20 years to make a vaccine,” Foley explains. “In an emergency situation, they could push it to 2-4 years. That’s still a long time in the event of an outbreak.”
And it’s even longer if you think that Zika may only be the tip of the iceberg of a phenomenon we are bound to see over and over again in the near future.
“The distribution, transmission, and abundance of vectors that bear and transmit diseases are being enhanced by global warming,” Foley and colleagues state in a recent publication . “The mean global temperature increased approximately by 1° centigrade during the last several hundred years. However, during the next 20 years it is anticipated to increase by 2–3° centigrade.”
Geographic areas that used to be too cold for mosquito-borne diseases are now seeing an increase in encephalitic viruses, dengue, and West Nile. Zimbabwe and Ethiopia are experiencing an increase in typhoid and cholera due to the same reason: combine poor hygiene with stagnant water and climate change, and you have the recipe for disaster.
So yes, a vaccine can provide a solution. But if this is only the beginning, we need to think globally. It’s not just one virus we’re fighting but a global change that’s happening too fast for the natural world to adapt on its own.
 Paul Shapshak , Charurut Somboonwit, Brian T. Foley, Sally F. Alrabaa, Todd Wills, John T. Sinnott (2015). Zika Virus. Global Virology I - Identifying and Investigating Viral Diseases Springer-Verlag