Over two days in early February, the World Health Organisation (WHO) convened an expert committee at its Geneva headquarters to consider the unthinkable.

The goal was to identify pathogens with the potential to spread and kill millions but for which there are currently no, or insufficient, countermeasures available. As the meeting opened, the city’s eponymous lake reflected a crisp blue winter sky. Only as the meeting progressed did an icy rain set in.

It was the third time the committee, consisting of leading virologists, bacteriologists and infectious disease experts, had met to consider diseases with epidemic or pandemic potential. But when the 2018 list was released two weeks ago it included an entry not seen in previous years.

In addition to eight frightening but familiar diseases including Ebola, Zika, and Severe Acute Respiratory Syndrome (SARS), the list included a ninth global threat: Disease X.