The World Health Organization (WHO) is preparing to fight the mysterious Disease X, a potential source of the next major international epidemic.

There’s just one catch: Disease X doesn’t exist yet, in humans anyway.

In the WHO’s own words:

“Disease X represents the knowledge that a serious international epidemic could be caused by a pathogen currently unknown to cause human disease.”

The organization included Disease X, along with seven other diseases in its 2018 R&D Blueprint, a guideline for identifying and prioritizing pathological threats that represent future epidemic risk, as well as major blind spots for treatment and vaccination.

Some of these diseases you’ve probably heard of — Zika and Ebola —and some you almost certainly haven’t — Rift Valley fever, Lassa fever, for example. The full list also includes Crimean-Congo hemorrhagic fever, Middle East respiratory syndrome coronavirus, Nipah virus, and severe acute respiratory syndrome (SARS).

In the case of many, but not all, of the diseases identified in the blueprint, there is no known medical treatment.

“For some of these diseases, there is not much research; there is some, but the main ones, we have nothing. We have no big initiatives around each of these,” Dr. Bernadette Murgue, who oversees the WHO’s R&D Blueprint, told Healthline.

“These roadmaps will identify priority areas; that could be the development of broad-spectrum antivirals, or vaccine technology... and also to increase laboratory capacity or regulatory readiness,” she said.

The task for scientists fighting against Disease X is, in essence, to prepare for the unknown, to brace ourselves against it using the knowledge and resources at hand.

For such an undertaking, the WHO has developed an incredibly thorough methodology to determine which diseases represent a potentially perfect storm of epidemic potential and absolute lack of effective treatment.

The original list of potential threats for this year’s blueprint was drafted based on the guidance of WHO disease experts around the world. A longer list that included well-studied diseases such as plague and cholera was first put together before culling the final 8-item disease blueprint that the organization published last month.

To prioritize certain diseases, the WHO evaluates many factors including human transmissibility, fatality rate, difficulty of detection, and the potential scope of an outbreak.

At first glance, the WHO blueprint seems to be lacking some diseases with a known serious potential to endanger human life on a global scale.

Antimicrobial-resistant strains of bacteria — those which have evolved so that traditional antibiotics no longer affect them — are an impending threat to global health.

Yet, they are nowhere to be found on this year’s blueprint.

“Remember what is the purpose [of the blueprint],” Murgue told Healthline, “which is to focus on diseases that have epidemic potential and for which we have nothing, no drugs, no vaccines, or an extensive research pipeline.”

“So, for a disease... like antimicrobial resistance, there are big initiatives. We didn’t want to be redundant, but rather to pick up what is more specific and on which we can focus our efforts. It doesn’t mean that these diseases are not important, but they are supported already by big initiatives,” she said.

The WHO also lists HIV, malaria, tuberculosis, smallpox, and cholera as known dangerous pathogens, but which do not fit within their criteria for the blueprint because of existing funding and infrastructure dedicated to them.

Disease X represents an area of study where not only could we have no real understanding of it, but zero research and development infrastructure either.

However, the WHO has ways of preparing for such an event.

While we may not know the specifics of Disease X, we almost certainly know something about where it will come from. By utilizing expertise in virology, bacteriology, and mycology (the study of fungus), scientists can create a framework to deal with a potential threat based on the wealth of knowledge already available about these pathogens.

The WHO also emphasizes “platform-based” approaches to dealing with diseases in their blueprint — that is, grouping diseases based on similarity. This maximizes the capacity for crosscutting research that can be more broadly applicable, rather than to just one specific disease or strain.

Zika, yellow fever, and West Nile virus are all, for example, part of a class of viruses known as flaviviruses. Understanding that they are related makes it easier to understand and treat the group as a whole.

And while the task at hand for the WHO and governments around the world to prevent the next major epidemic is daunting, Murgue is confident in their efforts.

“It took a bit of time, but now I must say we are very optimistic in a way. If you read what has been published recently about the blueprint and all the work around research and development, all that stuff is increasing more and more,” she said.