The World Health Organisation has added a new threat to its list of deadly viruses. On its list published in February of this year, #Disease X features for the first time. Though this is, in fact, a disease not identified as yet, the WHO felt it necessary to include a "known unknown." There, it joins #Deadly Diseases such as the Zika virus, Ebola, and SARS (severe acute respiratory syndrome).

What is Disease X?

The WHO uses the term Disease X for a "known unknown" pathogen with the potential to kill millions of people across the globe. When it comes to its origins, the WHO has identified a biological mutation, biological weapons, a terror attack or an accident as the source of this deadly disease.

Disease X could also be a zoonose like HIV, salmonella, and Ebola, all of which originated in animals but crossed over the humans in the 19th and 20th century.

These diseases have killed and will continue to kill millions of people because of their resistance to many drugs and the lack of effective treatments available to counter them. Much like Ebola and SARS, Disease X could also pose a global health threat.

Disease X is only different because it hasn't emerged or been identified yet.

Why include Disease X on the deadly disease list?

So, why include a disease that doesn't exist? The WHO has done so to raise awareness of the potential threats human life is subject too. Furthermore, the organisation hopes to put measures in place to prepare for and combat against the outbreak of a new virus.

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Speaking to The Telegraph, scientific adviser to the WHO and chief executive of the Research Council of Norway, John-Arne Rottingen, said that the next big outbreak is likely to be something not seen before. He added that it may seem unusual to add a Disease X to the list. However, he stressed that this was to "make sure we prepare and plan flexibility in terms of vaccines and diagnostic tests." Developing 'plug and play' platforms suitable for combatting a variety of diseases is believed to be key. Rottingen also emphasised the need to develop "systems that will allow us to create countermeasures at speed."

Where could a new deadly pathogen come from?

The most likely source of Disease X would be a zoonotic transmission, where a disease spreads from an animal to a human. HIV and Salmonella both originated in animals and spread to humans, causing very serious global health issues. Changing ecosystems and human habitats have the potential to see a transfer of animal diseases to humans.

In addition, Disease X could emerge and spread through the use of biological weapons. Throughout history, many warring factions have used viruses. The USSR and the USA worked on developing deadly pathogens during the Cold War. Stocks of smallpox and other dangerous pathogens still exist in secure laboratories. Under Saddam Hussein, the Iraqi military is believed to have experimented with botulinum toxins, while Al Qaeda toyed with anthrax.

More recently, biological weapons were used in Syria against the civilian population, while anthrax antibodies were identified in the blood a North Korean military defector. According to The Telegraph, experts now fear that Pyongyang is in possession of deadly anthrax weapons.

The most serious threat comes from known diseases

Although the WHO has chosen to include Disease X on its list of deadly diseases, experts believe that already known pathogens are growing ever more virulent. HIV, TB, and influenza still cause a major threat, killing thousands across the globe each year. Both the swine and the avian flu pandemics exposed the vulnerability and highlighted just how virulent viruses can be.

TB and HIV have been seen to be growing ever-more drug-resistant. Virus mutations such as these are still the greatest source of concern, despite the inclusion of Disease X.