"History tells us that it is likely the next big outbreak will be something we have not seen before", said John-Arne Rottingen, chief executive of the Research Council of Norway and a scientific adviser to the WHO committee.
"It may seem strange to be adding an 'X' but the point is to make sure we prepare and plan flexibly in terms of vaccines and diagnostic tests. We want to see 'plug and play' platforms developed which will work for any, or a wide number of diseases; systems that will allow us to create countermeasures at speed."
Advances in gene editing technology, which make the manipulation or creation of entirely new viruses possible, mean that Disease X could emerge through an accident or act of terror.
This week's brazen use of nerve gas in Salisbury underlines just how far the 100-year taboo on the use of chemical and biological weapons has broken down. And the creation last year in Canada of horsepox - a close relative of the deadly Smallpox virus - has heightened fears among experts that Disease X could come out of a renegade laboratory.
Mr Rottingen said a man-made Disease X was less likely than a naturally occuring one but warned: "Synthetic biology allows for the creation of deadly new viruses. It is also the case that where you have a new disease there is no resistance in the population and that means it can spread fast".
More likely, said Mr Rottingen, Disease X could be be sparked by a zoonotic disease - one that jumps from animals to humans - and then spreads to become an epidemic or pandemic in the same way H1N1 Swine flu virus did in 2009.
Zoonotic infections that have wreaked havoc in the past include HIV, which is believed to have jumped from chimpanzees to humans through the eating of bushmeat and has killed 35 million people since the early 1980s. Ebola is also a zoonosis.
"As the ecosystem and human habitats change there is always the risk of disease jumping from animals to humans", said Mr Rottingen. "It's a natural process and it is vital that we are aware and prepare. It is probably the greatest risk."
Professor Marion Koopmans, head of the Viroscience department of the Erasmus Medical Center in Rotterdam and also a scientific adviser to the WHO, said the frequency at which zoonotic diseases were appearing was becoming much more rapid.
She added: "The intensity of animal and human contact is becoming much greater as the world develops. This makes it more likely new diseases will emerge but also modern travel and trade make it much more likely they will spread."
The WHO developed a special tool or algorithm for determining which diseases and pathogens to prioritise each year. Diseases considered for inclusion must meet two key criteria: they must have the potential to cause a public health emergency through epidemic spread and there are no, or inadequate, countermeasures available.
Others diseases included on the newly released 2018 list include Crimean-Congo hemorrhagic fever; the zoonotic respiratory diseases MERS and SARS; and Zika which has been linked to severe birth defects in almost 30 countries.
MERS, SARS and Zika - all relatively new diseases - would in the past have come under the heading Disease X.
The list is designed to focus minds and spur development of new tests, vaccines and treatments. The WHO also hopes it will drive governments across the globe to invest in strengthening local health systems. This is important, as it is primary care systems - local doctors and nurses - that provide the best bet of detecting the outbreak of a new disease early and containing it before it is able to spread.
"We have these outbreaks and the international community flies in but in the case of Ebola the disease went under the radar for four months", said Professor Koopmans. "It's really crucial we start to change that and make sure local health care infrastructure is better developed.
"People on the ground are vital. They are our first line of defence".
Diseases threatening a public health emergency
• Crimean-Congo haemorrhagic fever (CCHF)
• Ebola virus disease and Marburg virus disease
• Lassa fever
• Middle East respiratory syndrome coronavirus (MERS-CoV) and Severe Acute Respiratory Syndrome (SARS)
• Nipah and henipaviral diseases
• Rift Valley fever (RVF)
• Zika
• Disease X
*Diseases posing significant risk of an international public health emergency for which there is no, or insufficient, countermeasures.
Source: World Health Organization (WHO), 2018
Notable plagues from history
• Plague of Athens (430 BC): One of the first recorded, the Plague of Athens killed a quarter of the city's population. The disease that caused it is still unknown.
• Black Death (1347 to 1353): The Black Death, caused by bubonic plague, killed some 50m people in Europe in less than a decade.
• First Cholera Pandemic (1817 to 1824): The death toll from the first cholera pandemic is unknown but from its origins in India, cholera spread worldwide, killing tens of millions. There is currently a serious outbreak in Yemen.
• Spanish Flu (1918 to 1919): Caused by the H1N1 virus, Spanish flu spread worldwide infecting a third of the global population. It killed between 50m and 100m people.
• Ebola (2013 to 2016): The virus primarily affected West Africa and killed over 11,000 people. It was started after a bat bit a one-year-old boy in Guinea.
