In just a few weeks, the Zika virus has gone from relative obscurity to a major concern of the World Health Organization (WHO), the Centers for Disease Control and Prevention (CDC), and other giants in global disease control. What’s happening with Zika, and why has it exploded now?

1. WHAT IS ZIKA VIRUS?

Zika is a virus transmitted by mosquitoes. It’s in the same family as several other mosquito-borne viruses that also can cause human disease, including yellow fever, dengue, and chikungunya. There is no treatment or vaccine available.

2. WHAT ARE THE SYMPTOMS OF INFECTION?

Most people who are infected with Zika virus don’t even know it; as many as 80 percent of the cases are asymptomatic. For those who do show symptoms, fever, headache, rash, and joint and muscle aches are the most common signs of infection.

3. HOW DOES THE VIRUS SPREAD?

Zika is an arbovirus—a virus that is “arthropod-borne.” It is transmitted by mosquitoes—most commonly a mosquito called Aedes aegypti. This mosquito lives mainly in tropical areas of the world, including parts of the United States. A. aegypti is well-adapted to live with humans, laying eggs in stagnant water that can be found around homes—old tires, bird baths, cans, or pots. They feed almost exclusively on humans and can be day-biters, so some interventions to keep mosquitoes away, such as bed nets, aren’t as helpful. They also travel well and can spread via boats or potentially airplanes that move around the world.

While A. aegpyti is the main vector for Zika, there is concern that other mosquitoes may be able to transmit the virus. Another invasive mosquito, the Asian tiger mosquito (Aedes albopictus) is a common disease vector. It can spread viruses related to Zika, and was the main vector of a large Chikungunya outbreak in 2006. This mosquito also was involved in a Zika outbreak in Gabon in 2007, but that was a different strain of Zika than the one currently circulating in the Americas. Researchers in Brazil are also testing the possibility that another mosquito species of the Culex family could spread Zika. While A. aegypti is geographically limited, A. albopictus and various species of Culex mosquitoes are more widely distributed, living in colder climates.

Human-to-human transmission is also theoretically possible. There are two reports of Zika virus transmission via sex. One report involved an American who had contracted Zika in Senegal and developed symptoms upon his return to Colorado, and whose wife developed Zika infection after exposure. A second case found Zika virus in the semen of a man in Tahiti long after the virus had been cleared from the blood. However, even if this is possible, it seems to be a rare mode of transmission.

4. WHERE DID IT COME FROM?

We’ve known about Zika since 1947, when it was discovered in a monkey in the Zika forest in Uganda. A year later, the virus was found in a mosquito in the same location, and blood samples from humans showed antibodies to Zika, evidence they had previously been infected with the virus. The first documented human case was identified in Nigeria in 1968. Blood testing in the 1950s and 1960s showed infection with the virus was widespread in humans across Africa and many parts of Asia. The first outbreak of Zika outside of Africa or Asia was on Yap Island in 2007, infecting almost 75 percent of the island’s population of 6900 people. A second outbreak in French Polynesia in 2013–2014 may have infected as many as 19,000 people.

5. HOW WIDESPREAD IS IT?

Zika first appeared in Brazil in mid-2015, possibly introduced from French Polynesia during 2014’s World Cup soccer tournament or other international sporting events. Active transmission of the virus has been confirmed in 22 countries as of January 28. Many of them are popular travel destinations.

Because they are tourist spots, individuals infected during travel have returned home, incubating the virus. Travel-associated cases have been diagnosed in Denmark, Spain, Portugal, Italy, and the United Kingdom. The United States has seen approximately a dozen cases of imported Zika infections. No local spread has been documented in these countries. The WHO has suggested the outbreak could eventually affect up to 4 million people; approximately 1 million have already been infected.

6. WHY IS THIS OUTBREAK SO SERIOUS?

First, the outbreaks are very large and have infected large proportions of the population where they’ve occurred. Because this virus has never been identified in this geographic location before, the population has no immunity to it—meaning everyone is vulnerable.

In Brazil, there has been an increase in reports of microcephaly in babies. This means that the babies are born with an abnormally small head, and problems with brain development. This has been linked to Zika virus infections in the mother during pregnancy; almost 4000 babies have been diagnosed with the condition since October 2015. A baby born in Hawaii to a mother who had been infected with Zika was also born with microcephaly. As a consequence, the CDC has issued interim guidelines for travel during pregnancy, suggesting that pregnant women do not travel to areas experiencing a Zika outbreak. Other countries experiencing outbreaks have suggested that women delay pregnancy for months or years, until the outbreak subsides—a difficult proposition in an area where many pregnancies are unplanned and access to birth control is limited.

Increases in another neurological condition, Guillain-Barré syndrome, has also been reported in Colombia, and that is thought to be due to Zika virus infections. Though microcephaly has not been reported in association with previous Zika outbreaks, Guillain-Barré syndrome was identified in the 2013 outbreak in French Polynesia.

7. WHAT DON'T WE KNOW ABOUT ZIKA?

A lot. Right now, there appears to be a correlation between microcephaly and Zika infection, but we can’t be 100 percent sure that Zika is causing it—or that there really is an increase in microcephaly cases at all. If Zika is indeed causing microcephaly (and/or Guillain-Barré syndrome), we have no idea how the virus is doing this. Assuming the virus is responsible, we don’t know if infection needs to be in a certain developmental window during the pregnancy, or if the infection would have to be symptomatic in order for microcephaly to result. We have no vaccine or treatment for Zika, though the outbreak has spurred interest in developing them. For now, control efforts are concentrated on mosquito elimination and education of the population about the potential risks of infection.