What is disease X and what are the manifestations?

Whether caused by viruses, bacteria or parasites, in recent decades, over 70% of emerging infectious diseases have been of animal origin (known as zoonoses).

These epidemics can be caused by an unknown pathogen (case X disease) or by an already identified pathogen that has conquered a new geographical area, or that has been modified to give birth to a new variant.

Studying and detecting areas with the highest risk (also known as hotspots) is difficult, because the way zoonoses spread depends on the spatial distribution of their reservoirs (wild or domestic animal species that harbor the pathogen without developing symptoms of of the disease), as well as of the mammalian hosts (species that can be contaminated and can develop the disease) and their interactions with humans.


  1. Disease X – probable symptoms
  2. 3 large families of viruses that can cause disease X
  3. Where are the areas at risk of disease X?
  4. The very special case of South America
  5. Disease X will not happen by chance


Disease X – likely symptoms

Therefore, disease X is an unknown, hypothetical, but contagious and fatal pathology.

For the WHO, the pathogen X in disease X could be a zoonosis, that is, transmitted by animals or insects. Therefore, it is conceivable that this hypothetical virus could cause symptoms typical of zoonotic diseases, such as:

Respiratory symptoms:

  • cough;
  • breathing difficulties;
  • sore throat.

General symptoms:

  • fever;
  • chills;
  • muscle and joint pains;

Gastrointestinal symptoms:

  • nausea,
  • vomiting,

Neurological symptoms:

  • headaches;
  • confusion;
  • anosmia (loss of smell) and ageusia (loss of taste).

Skin symptoms:

  • skin rash.

Organ-specific symptoms:

  • they can vary depending on the organs affected, such as the liver, kidneys or heart.

3 large families of viruses that can cause disease X

For the moment, the institution estimates that between 631,000 and 827,000 unknown viruses have the ability to infect humans and that pathogen X could be one of them.

Among the future presumed culprits of disease X: three large families of viruses. Although it must be taken into account that certain bacteria can also be responsible for the occurrence of diseases, between 2020 and 2024 viruses were mainly responsible for major epidemic outbreaks.

If we look at our recent past, we know that the non-sexually transmitted viruses that have appeared in recent decades and that have presented the greatest infectious risks for humanity belong to three large families of viruses: Filoviridae (Ebola, Marburg etc.), Coronaviridae (SARS-CoV-1 and SARS-CoV-2, MERS, etc.) and Henipaviruses (Nipah virus, Hendra virus, etc.).

Therefore, it is very likely that disease X is related to a virus that belongs to one of these 3 large families. There is no certainty, but, considering the experience of the last decades, the probability that a zoonotic disease will arise from a family of viruses that has never been involved in epidemics is relatively low.

Where are the areas at risk of disease X?

Hot spots for Filoviridae diseases are found in Africa, in the forest regions of Uganda, South Sudan and the eastern parts of the Democratic Republic of Congo, with smaller areas in western and central Africa, as far south as Angola.

In Africa, the variables associated with these occurrences could be related to human behavior, such as the consumption of wild animal meat, which is often associated with Ebola virus epidemics, loss of biodiversity or even other bioclimatic covariates.

The regions with a high risk of infectious diseases caused by Coronaviridae predominate in the Indian subcontinent, with a few areas in China and Southeast Asia. These regions have been identified since 2019.

Finally, hotspots for Henipavirus disease are spread along the west coast of India, in Bangladesh, along the coast of Malaysia and in small areas of the Indonesian archipelago.

If we summarize this information regarding the three large families of viruses responsible for the great epidemics of the last decades, we find that Uganda and part of China are regions of the world where the socio-media and climatic conditions are present for disease X to occur.

The very special case of South America

The number of major epidemics increased more than ten times between 1940 and today. All of them were caused by a pathogen that appeared on the African and Asian continents, despite the fact that localized epidemic outbreaks periodically appear throughout the world.

A paradox emerges from this observation, namely the fact that South America did not contribute to these major outbreaks of zoonotic diseases. Although this continent hosts the richest biological diversity on our planet, with approximately 60% of the world’s terrestrial life (as well as an extremely varied marine and freshwater flora and fauna).

The Amazonian rainforest itself is an enormous reservoir of viruses and bacteria, as is the diversity of hosts and habitats it harbors. So why aren’t we seeing more outbreaks in these regions?

A possible explanation is the existence of a dilution effect that is still very present in Amazonia, which remains relatively protected compared to the forests of Indonesia and Cameroon. Factors related to the sociology of local populations could also come into play.

Disease X will not happen by chance

The science of data (and especially biogeography) tells us that the emergence of disease X that strikes our species will not happen by chance. It is likely that disease X depends on environmental factors, such as changes in the landscape (especially the loss of tree cover) and climatic variations.

Using a biogeographical approach and satellite images, it is possible to identify the potential hotspots where disease X could appear. Therefore, there is no need to think of any conspiracy when such an event is predicted


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