Emergences from yesterday's revisited : plague and cholera

The plague

Three major pandemics have been clearly documented, in particular through the analysis of fossil DNA (or ancient DNA = aDNA) taken from skeletons in archaeologically validated necropolises. The first pandemic was Justinian's plague (541-544, also known as pestis inguinaria or pestis glandularia), which lasted intermittently until around 750. The second pandemic, the European Black Death (1347-1351) claimed the lives of 30-50% of Europe's population, and led to the economic and social collapse of cities and regions, as well as a moral and religious crisis. It was followed by successive waves, the Great Plague of London in the 17th^century and the plague of Marseille in the 18th^century being only late replicas of the Black Death and not reimportation events from Asia, as fossil DNA analysis attests. The third pandemic emerged in China at the end of the 19th^century , exploded in 1894 (discovery of Yersiniapestis by Alexandre Yersin in Hong Kong) and spread across the globe until the middle of the 20th^century . In addition to molecular diagnosis, which is "relatively easy" because it does not require knowledge of the entire genome, it is now possible to develop true phylogenetic genomics, which, on the basis of advanced sequencing of the aDNA sampled, makes it possible to reconstitute Y. pestis genomes from different eras and pandemics, to compare them and thus draw conclusions on the occurrence of genomic modifications that may have led to the emergence of, or explain the particular severity of, an epidemic or pandemic.

Cholera

Throughout history, cholera has struck across the globe. Hippocrates (460-377 BC) and Galen (129-216 AD) described a disease closely resembling cholera. The disease has been known since antiquity in the Ganges delta, the historical "epicenter" of cholera, and is rooted in the Indian subcontinent. The first modern descriptions by British troops as early as 1800 report violent epidemics affecting hundreds of thousands of individuals, causing very high mortality. One of the most feared diseases in history, it became pandemic in the 19th^century following the explosion in land and, above all, sea trade. However, it wasn't until the second half of the 20th^century that the basis of Vibrio cholerae 's pathogenicity in humans was elucidated, following its identification by Robert Koch. Rita Colwell's work also made it possible to revisit the epidemiology of cholera, and to move away from a purely anthropocentric view of the disease towards the global concept of a microorganism whose natural environment corresponds to warm, semi-saline estuarine waters (such as those of the Bay of Bengal), whose density varies according to that of the phytoplankton of which V. cholerae is a commensal. Cholera as a disease is in fact only an "epiphenomenon" in relation to the environmental lifestyle of vibrio cholerae.