Where do these new viruses come from?

Advances in hygiene and health allowed, during the XXe century, to roll back many infectious, bacterial, parasitic or viral diseases, or even to eradicate some, like smallpox in 1979, following vaccination campaigns. And yet, despite this progress, new diseases are still emerging, or old diseases are re-emerging. Where do the pathogens that cause such infections come from – or come from?

If the names of Zika, chikungunya, Ebola, Nipah you are familiar with is that virus for which they are responsible were – or are – responsible forepidemics in different places of the planet, even on the whole planet (pandemic).

Some of these viruses and the infection they cause were already known in the past, but either reappeared after a long absence, or appeared in regions where they did not rage, hence the term old re-emerging diseases. Such is the case with the virus chikungunya : known since the early 1950s in Africa. He has contaminated the populations of the Indian Ocean in 2004-2005, then those of the Caribbean and the American continent in 2013, to reach Oceania in 2014. The coronavirus, responsible for SARS (syndrome severe acute respiratory disease), qualifies as emerging this disease because the virus was unknown inspecies human before 2002-2003 (Vietnam, China). Theemergence therefore qualifies the appearance of a new pathogen in a host here, Man.

But where do these viruses come from and how do they reach humans?

These viruses do come from somewhere, however! Before moving to humans, they most often infect one or more animal species, which constitute the tank natural virus, the animal can be asymptomatic carrier. When an emerging disease appears, it is because a pathogenic agent has on the one hand come into contact with the new host and on the other hand that it has succeeded in crossing what is called the species barrier; it has become able to multiply in its new host and to spread within this new population.

So the HIV-1, responsible for AIDS, has gone from monkeys to humans and the Ebola virus (the cause of fever eponymous hemorrhagic and at an estimated death rate of 70% in West Africa) has for reservoir Bats, the monkey possibly being an intermediate host.

The first key element of emergence is therefore the bringing together of humans with animal species, reservoirs of virus, from which they were previously separated. The deforestation, by the destruction of their natural habitat, brings into contact with humans species that are usually far from it; periods of famine can also lead individuals, to survive, to hunt unusual prey, small rodents for example, which are the reservoir of viruses such as the Sabia virus, Chapare, Guanarito. The latter, responsible for fever Venezuelan hemorrhagic, can be transmitted by simple inhalation dust contaminated with stool or urine from an infectious animal! The global warming, the communication channels between distant countries can bring species ofinsects blood-sucking, vectors of diseases like dengue or Chikungunya, to colonize new environments.

The second key element in the emergence lies in the capacity that the virus to infect and replicate a cell in the new host. The plasticity of genome viral, due to phenomena of mutations, recombinations or reassortments of Genoa (coming from two strains of the same virus infecting the same host), can lead to the appearance of a new variant genetic replicating effectively in its new host and also transmitting more easily between individuals. Thus, the virus has adapted to its new host, which can become more virulent and contagious. Man thus becomes a new reservoir for this variant.

Such a succession of events leading to the effective adaptation of a virus to a new host is a natural but rare phenomenon (and certain viruses are not subject to this adaptation phenomenon); however, its probability increases with changes in ecosystems for which humans are responsible (deforestation, human gatherings, globalization of trade, tourism, etc.).

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