On 1 February 2016, I declared that the Zika outbreak sweeping through the Americas was a public health emergency of international concern. That was not an easy call. But looking back, it was the right one.

At that time, the disease itself, long dismissed as an obscure medical curiosity, could hardly be described as “extraordinary”, which is the principal requirement for declaring an international health emergency. In the decades between its discovery in Uganda in 1947 and its appearance in the Americas, only a few human cases of Zika virus were reported.

The 18 international experts who advised me in the Zika emergency committee had additional, though inconclusive, evidence to draw on. In 2007, Zika left its ancestral home to cause its first outbreak, on Yap Island in the western Pacific Ocean. That outbreak was surprising, but ultimately reassuring. Although almost three-quarters of the population were infected with Zika virus, only about 1000 people fell ill with sickness attributable to the virus. None of the cases required hospitalization, and the outbreak ended after just three months.

"Like every other explosive outbreak, Zika revealed fault lines in the world’s collective preparedness." Dr Margaret Chan, Director-General of WHO

The next surprise was more ominous. Having demonstrated its ability to spark an outbreak, Zika did so again in French Polynesia from 2013-2014, causing an estimated 30,000 cases. Though all cases were mild, doctors were puzzled by a disturbing uptick in cases of Guillain-Barré syndrome, a severe and usually rare neurological complication. Was this just a coincidence, or was something more sinister at work?

By the start of 2016, nearly everyone had seen the heart-breaking images of babies born in Brazil with tiny heads. We all heard the tragic stories of their distraught mothers and the bleak outlook projected for their babies. The possibility that a mosquito bite during pregnancy could cause severe neurological damage in babies deeply alarmed the public, but also astonished scientists. They asked: Why only now, and why only in Brazil?

At the time, Brazil was also experiencing large outbreaks of dengue and chikungunya. Could the three viruses somehow interact, in an amplifying way, to damage babies in the womb? Could something in the environment of northeastern Brazil, the epicentre of the outbreak, be partly responsible, perhaps a chemical or a natural toxin? No one had firm answers.

Fortunately, experts on the emergency committee could draw on some brand new evidence. In an elegant piece of detective work, a retrospective investigation of the outbreak in French Polynesia unearthed findings strongly suggesting a link between Zika infection during pregnancy and microcephaly in newborns. Now it wasn’t “only Brazil” anymore.

A year ago, when I declared an international health emergency, it was this suspected link between Zika infection and microcephaly and other neurological complications that, according to my advisers, turned the outbreak into an “extraordinary” event.

Zika also satisfied two further criteria relevant to declaring a public health emergency of international concern. As few populations had any immunity to this previously rare disease, the virus could spread, unchecked, like wildfire, sparked by the volume of international air travel. Any area that hosted the competent mosquito species, Aedes aegypti, was considered at risk – a geographical area estimated to comprise nearly half the world’s population. With so many people at risk and so few control tools, the outbreak clearly required a coordinated international response.

Like every other explosive outbreak, Zika revealed fault lines in the world’s collective preparedness. Poor access to family planning services was one. The dismantling of national programmes for mosquito control was another.

One year later, where do we stand? International spread has continued, while surveillance has improved. Some 70 countries and territories in the Americas, Africa, Asia, and the Western Pacific have reported cases since 2015. The documented consequences for newborns have grown to a long list of abnormalities known as “congenital Zika virus syndrome”. We know that the virus can be sexually transmitted, adding further precautionary advice to women of childbearing age.

In terms of prevention, we may not remain so empty-handed for long. In line with WHO advice, some innovative approaches to mosquito control are being piloted in a number of countries, with promising results. Some 40 candidate vaccines are in the pipeline. While some have moved into clinical trials, a vaccine judged safe enough for use in women of childbearing age may not be fully licensed before 2020.

In November 2016, I lifted the declaration of Zika as a public health emergency of international concern, again on the advice of the expert committee. That, too, was the right call. By then, research had addressed many of the questions that made the disease so “extraordinary” nine months earlier. Some uncertainties remain, but many fundamental questions have been answered.

In large parts of the world, the virus is now firmly entrenched. WHO and affected countries need to manage Zika not on an emergency footing, but in the same sustained way we respond to other established epidemic-prone pathogens, like dengue and chikungunya, that ebb and flow in recurring waves of infection. That is why WHO is creating a cross-Organizational mechanism to provide sustained guidance for effective interventions and support for families, communities, and countries experiencing Zika virus. For the research community, WHO has identified priority areas where more knowledge is urgently needed.

We are now in the long haul and we are all in this together. WHO's strategic planning and commitment to work with partners for sustained interventions and research should go a long way towards bracing the world for this challenging – and still heart-breaking – effort.