Every year, the World Health Organization commissions an expert committee identify the most threatening infectious diseases of the upcoming year. The idea is to prioritize research and development on diseases and pathogens that pose a major risk to global health, but lack effective treatments or vaccines.

The committee met early in February this year, and the prioritized list of diseases has been released. The list is made up of familiar threats, including Ebola, Zika, Lassa Fever and a respiratory illness in the Middle East known as MERS.

And then there’s “Disease X.” It is the last on the list, and most mysterious.

What is Disease X?

Disease X is quite literally a mystery disease. It’s a recognition that we can’t see everything coming. In 2018, it’s entirely possible that we’ll see a brand-new pathogen. Or, as with Zika, an old disease will suddenly demonstrate a new way to harm us.

Disease X is a placeholder for disaster we can’t imagine yet.

New diseases appear all the time. Deadly Nipah virus appeared in Malaysia in the late 90s; we have no prior evidence of the disease. Severe fever with thrombocytopenia syndrome appeared in China in 2009, mostly likely carried by a tick from a wild animal reservoir. Heartland virus, another tick-borne pathogen with a wild animal reservoir, was first isolated in the US in 2009. Disease X could be one of these such diseases.

Will Disease X come from animals?

While there are a lot of possible sources for Disease X, one very likely reservoir of pathogens is the zoonotic disease. These are disease present in animals – wild or domestic – than can also be transmitted to humans. HIV was originally a zoonotic disease, which probably transmitted to humans for the first time when someone killed and ate a wild chimpanzee. HIV was present in chimpanzees long before it made the jump to humans – at some point the virus evolved to infect us well. Ebola virus disease is also a zoonosis; the most recent pandemic began when a one-year-old boy in Guinea was bitten by an Ebola-infected bat. Approximately 70% of new diseases are zoonotic.

One candidate for disease x could be Brucellosis. This is a bacterial infection that’s a lot like tuberculosis and is prevalent in an estimated 10% of farmed dairy cattle around the world. Humans are infected when they eat dairy products from infected animals. Right now, it’s kept in check by testing of commercial dairy products and cattle vaccination, and it doesn’t spread among people. Raw milk consumption, and a minor bacterial mutation could change that.

Avian influenza is a similar case. It can already be transmitted by birds to humans, but it doesn’t spread human-to-human — not yet, at least. There are plenty of flu viruses that do spread person-to-person, though, so it is probably just a matter of time before avian influenza evolves to do it. Avian influenza and brucellosis, or other domestic livestock diseases, then, could be Disease X.

Disease X could also be a previously unknown pathogen from an animal reservoir. Human beings are pushing into the last wild spaces on the planet, and those wild places also contain new diseases. Farming the rainforest or developing the jungles of Madagascar means exposing humans to diseases we’ve never met before. In 1999, Nipah virus killed 109 people in Malaysia – infected fruit bats infected pigs which infected people – and we’d never even heard of the virus before 1998. Disease X could be an utter wild card like Nipah, a hemorrhagic virus or virulent airborne bacteria previously unknown to global health.

Will Disease X come from people?

Humans have been using diseases as weapons since 1500 BC, when the Hittites sent people infected with plague into enemy territories. In recent history, both the US and the USSR experimented with bioweapons. One favorite among the Soviets was anthrax; an accidental release in 1979 killed 66 people. Anthrax can last for decades in storage and remain dangerous, and we don’t know how many former Soviet republics still possess poorly protected anthrax stockpiles.

Newer bioweapons are a threat, too. Just a year ago, North Korean leader Kim Jong-Un’s half-brother was killed by a biological nerve agent called VX. In December, a North Korean defector was found to have anthrax antibodies in his immune system, indicating he had probably been vaccinated for anthrax. Security observers are reasonably certain that North Korea develops secret bioweapons on an ongoing basis.

Disease X could come from a deliberate attack. It could be an act of war from a state entity using bioweapons developed for the purpose, or a terrorist attack from a group who was able to purchase a bioweapon on the black market.

How do we prepare for Disease X?

Disease X thinking is big thinking.

First, it admits that even the best tools we current possess cannot forecast every problem. We had absolutely no idea that Zika could cause microcephaly in pregnancies until 2015, even though the disease was first identified in Uganda almost seventy years prior.

To fight a disease outbreak – any disease outbreak – you need health care providers who can treat the disease, laboratories to diagnose the disease, and supplies and equipment to support diagnostics and treatment. Those things together make up a health system. Strengthen the health system means better preparedness for Disease X, no matter what X may be.

That means we prepare for disease X by using systemic approaches that make us better at fighting every disease. If we improve the skills of laboratory technicians in developing countries, and equip those laboratories with better equipment, we increase our global ability to diagnose and treat all diseases. If vaccine manufacturers are able to rapidly change their production lines from one kind of vaccine to another, we’re more prepared to fight a new pandemic. New technology can also help – the faster and closer to an outbreak we can start lag diagnostics, the more rapidly we can develop treatments, cures, and vaccines.