Ebola emergency simulation in Malpensa. A bogus infected patient arrives in the airport, and is carried to "Sacco Hospital" by a special modified ambulance. Photo: Shutterstock

In the wake of the Ebola outbreak in 2014, officials at the World Health Organization began to brainstorm what disease could pose the biggest threats on a global scale. They included Ebola and other germs for which there were little or no controls, like Crimean Congo hemorrhagic fever and Marburg and Lassa fever viruses.

The dreaded “Disease X” is the latest addition to the annual R&D Blueprint for priority diseases for 2018.

The health agency’s explanation: germs unknown to science could pose as big a risk as any catalogued pathogen, no matter how virulent.

“Disease X represents the knowledge that a serious international epidemic could be caused by a pathogen currently unknown to cause human disease,” write the scientists. “So the R&D Blueprint explicitly seeks to enable cross-cutting R&D preparedness that is also relevant for an unknown ‘Disease X’ as far as possible.” 

Indeed, some of the biggest killers in history appeared out of nowhere. For instance, the appearance of the bubonic plague in Europe starting in 1347 was originally thought by many to be a divine punishment – and its connection to poor sanitation and rat populations was not really understood for centuries. AIDS and its immune-deficiency complications were clinically observed in the late 1970s and early 1980s – but it took several more years to isolate the heretofore-unknown Human Immunodeficiency Virus, or HIV, as the underlying cause. New strains of influenza virus have appeared over the last century, some with pandemic results.

Zika virus now also appears on the WHO Blueprint list for the first time. The virus was originally discovered in 1947. But it wasn’t until 2015 that transmission really took off, infecting hundreds of thousands in multiple hemispheres, tagging it as an international public health menace. A WHO study published in the journal PLOS Medicine last January confirmed the virus’s role in several suspected conditions. The WHO scientists concluded Zika is causative in Guillain-Barre Syndrome, the birth defect microcephaly, and other congenital brain defects.

The other WHO Blueprint list of diseases remains largely the same: they include the aforementioned Crimean-Congo and Ebola and Lassa viruses, according to the review. Also still on the list is Middle East Respiratory Syndrome Coronavirus, or MERS; Severe Acute Respiratory Syndrome, or SARS; Nipah and henipaviral disease; and Rift Valley fever.

Left off the WHO list is the Severe Fever with Thrombocytopenia Syndrome, or SFTS.

Other germs that were considered as possible candidates, but which were not included as a Blueprint pathogen included Chikungunya, monkeypox and leptospirosis, among others.

Recommendations for action on the diseases include better diangostics, better research on transmission routes, and needs assessments of countermeasures for multiple germs, or families of the germs.