The infection has prompted the World Health Organization to declare a global health emergency due to the link to thousands of suspected cases of babies born with small brains - or microcephaly - in Brazil.
But there are still many, crucial, unanswered questions.
How many people have been infected in the Americas?
The best estimate of Zika infections is between 500,000 and 1.5 million - which is quite a wide margin of error. What percentage of people in an affected area are getting infected? Is it everyone? We don't know.
Why the explosive outbreak?
One theory is that the virus has mutated to become more infectious. Alternatively, some experts argue it could simply be a case of the virus reaching areas where people are densely packed together and there are huge numbers of mosquitoes.
Who is infectious?
Around 80% of people have no symptoms when they get the virus - although this figure needs further investigation. It's not known if they can also spread the virus or even why they are asymptomatic.
Does it cause microcephaly?
It's the biggest health concern in the outbreak, yet the link with Zika and birth defects is still only "strongly suspected". Parts of Brazil that have seen cases of Zika have, several months later, also seen a surge in microcephaly. However, the trials to prove the link have not finished.
How risky is infection?
If the virus does cause microcephaly, how often does this happen? Does every infection lead to birth defects? Or is it one-in-100? One-in-10,000 perhaps? At the moment it's not clear how worried pregnant women should be.
Is there a risky period in pregnancy?
If the virus causes microcephaly, does it matter when you are infected? There have been some suggestions that the first trimester (the first 12 weeks) is key, but other doctors have hinted there might be risks as late as 29 weeks. And those risks could change over time.
How could it damage the brain?
Some infections, such as rubella, can damage the brains of developing babies during pregnancy. But it is not known how Zika could be crossing the placenta and damaging brain growth.
Does developing symptoms change the risk?
Around four-in-five people infected will not develop symptoms. Do silent infections carry the same risks of microcephaly as those which result in a fever or a rash? There is also the rare neurological disease Guillain-Barre syndrome that has been linked to Zika infection and we don't know which patients are most a risk.
What's going on in Africa and Asia?
The virus was first detected in Africa and then parts of Asia until it reached Brazil and then spread. So do these continents represent giant vulnerable populations susceptible to Zika outbreaks? Or has Zika been around and undetected there for years, so that most of the population are immune? It is hard to establish the global threat without knowing the answer.
How big is the surge in microcephaly really?
There are big questions about the quality of the data, both before the outbreak of Zika and now. The figures for previous years may be underestimates, and the number of suspected cases is an overestimate. Of the 4,783 reported cases of microcephaly - 404 have been confirmed, 709 have been disproved and 3,670 are still being investigated.
Can it be spread by other mosquitoes?
The Zika virus is spread by the mosquito, but it is confined to tropical and sub-tropical countries. There is concern that Zika could be spread by other mosquitoes too such as the Asian tiger mosquito. This species prefers more temperate climes such as parts of Europe.
How big a risk is sex?
It seems likely that the overwhelming majority of cases are spread by the mosquito where the insect bites a person with the virus and passes it onto the next person it bites. But sexual transmission has been implicated in a couple of infections. It's unknown how common this is.
What about immunity?
Are you infected once and then protected for life, like measles? Or does it take multiple infections to achieve immunity? How long does immunity last? These answers will tell us how long the outbreak could last and indicate whether a vaccine would be effective.