Abstract
Emerging diseases can be classified according to the pathogen's mode of emergence. Three modes of emergence are described :
1) the crossing of the species barrier by a pathogen, most often a virus, which, once adapted to its new human host, is capable of triggering an epidemic. The most common examples are SARS and AIDS ;
2) the modification of the genome of an infectious agent which, as a result, acquires a new virulence or escapes the immune response that the host had developed against this infectious agent. This genome modification can take the form of reassortments, as in the case of the influenza virus, or mutations under drug pressure, in the case of viruses, bacteria, parasites or fungi ;
3) the arrival of an infectious agent in a geographical area where it was not previously circulating. This was the case for the West Nile virus, which first arrived on the American continent in New York in 1999, and for the chikungunya and Zika viruses, which have begun a world tour over the past decade, having previously been confined to Africa and Asia. Factors associated with the greater circulation of infectious agents are the considerable increase in the human population, which quadrupled in the 20th century ; the very high human densities reached within tropical megalopolises, often unhealthy ; and the increased mobility of populations, making it possible with the airplane to spread pathogens in a matter of hours to the four corners of the planet. Another major factor in the spread of disease is the Aedes mosquito, present in both tropical (Aedes aegypti) and temperate (Aedes albopictus) zones. This mosquito transmits several arboviroses that are veritable scourges, such as yellow fever, dengue fever, chikungunya and Zika. The global colonization begun by Aedes albopictus fifty years ago is set to accelerate under the influence of global warming, as demonstrated by the mosquito's progression in the southern half of France over the last ten years. Other ways in which infectious agents spread include the widespread use of injections and transfusions (transmission of AIDS and hepatitis B and C viruses), the global food trade (e.g. : importation of Egyptian fenugreek into Germany, the source of the Escherichia coli 0104:H4 epidemic in 2011) and the illegal trade in animals (e.g. : monkeypox epidemic in the USA following the clandestine importation of Gambian rats in 2003).
