From October 2015 to January 2016, there were almost 4,000 cases of babies born with microcephaly in Brazil. Before then, there were just 150 cases per year.

The suspected culprit is a mosquito-borne virus called Zika. Officials in Colombia, Ecuador, El Salvador and Jamaica have suggested that women delay becoming pregnant. And the Centers for Disease Control and Prevention has advised pregnant women to postpone travel to countries where Zika is active.

The World Health Organization says it is likely that the virus will spread, as the mosquitoes that carry the virus are found in almost every country in the Americas.

Zika virus was discovered almost 70 years ago, but wasn’t associated with outbreaks until 2007. So how did this formerly obscure virus wind up causing so much trouble in Brazil and other nations in South America?

Zika’s Origins

Zika virus was first detected in Zika Forest in Uganda in 1947 in a rhesus monkey and again in 1948 in the mosquito Aedes africanus , which is the forest relative of Aedes aegypti . Ae. aegypti and Ae. albopictus can both spread Zika. Sexual transmission between people has also been reported.

Zika has a lot in common with dengue and chikungunya, another emergent virus. All three originated from West and central Africa and Southeast Asia, but have recently expanded their range to include much of the tropics and subtropics globally. And they are all spread by the same species of mosquitoes.

Until 2007, very few cases of Zika in humans were reported. Then an outbreak occurred on Yap Island of Micronesia, infecting approximately 75% of the population . Six years later, the virus appeared in French Polynesia , along with outbreaks of dengue and chikungunya viruses.

Zika in the Americas

Genetic analysis of the virus revealed that the strain in Brazil was most similar to one that had been circulating in the Pacific .

Brazil had been on alert for an introduction of a new virus following the 2014 FIFA World Cup because the event concentrated people from all over the world. However, no Pacific island nation with Zika transmission had competed at this event, making it less likely to be the source.

There is another theory that Zika virus may have been introduced following an international canoe event held in Rio de Janeiro in August of 2014, which hosted competitors from various Pacific islands.

Another possible route of introduction was overland from Chile, since that country had detected a case of Zika disease in a returning traveler from Easter Island.

Sometimes Symptomless

According to research after the Yap Island outbreak, the vast majority of people ( 80% )infected with Zika virus will never know it—they do not develop any symptoms at all. A minority who do become ill tend to have fever, rash, joint pains, red eyes, headache, and muscle pain lasting up to a week. And no deaths had been reported.

However, in the aftermath of the Polynesian outbreak it became evident that Zika was associated with Guillain-Barré syndrome , a life-threatening neurological paralyzing condition .

In early 2015, Brazilian public health officials sounded the alert that Zika virus had been detected in patients with fevers in northeast Brazil. Then there was a similar uptick in the number of cases of Guillain-Barré in Brazil and El Salvador . And in late 2015 in Brazil, cases of microcephaly started to emerge.

At present, the link between Zika virus infection and microcephaly isn’t confirmed, but the virus has been found in amniotic fluid and brain tissue of a handful of cases.

How Zika might affect the brain is unclear, but a study from the 1970s revealed that the virus could replicate in neurons of young mice, causing neuronal destruction . Recent genetic analyses suggest that strains of Zika virus may be undergoing mutations , possibly accounting for changes in virulence and its ability to infect mosquitoes or hosts.

The Swiss Cheese Model

One way to understand how Zika spread is to use something called the Swiss cheese model . Imagine a stack of Swiss cheese slices. The holes in each slice are a weakness, and throughout the stack, these holes aren’t the same size or the same shape. Problems arise when the holes align.

With any disease outbreak, multiple factors are at play, and each may be necessary but not sufficient on its own to cause it. Applying this model to our mosquito-borne mystery makes it easier to see how many different factors, or layers, coincided to create the current Zika outbreak.