A barrier at the heart of infectious emergence: the species barrier

The first lecture, entitled "A barrier at the heart of infectious emergence: the species barrier", dealt with the most complex and probably least well-defined barrier of all. At a time when the emergence of new diseases and/or pathogens is such an acute phenomenon, the aim was to better understand how a pathogenic microorganism, most often adapted to one or more animal species, could "jump" and establish itself in the human species. Using well-studied models, the main stages in species hopping were reviewed and analyzed: conditions facilitating contact, appearance of reservoir animals in close contact with humans, sharing of primary receptors at the body's major barriers, in particular the respiratory mucosa, and replication capacity in the human host. These two properties are essential for successful viral emergence. However, their extreme complexity makes them difficult to model, as they involve numerous stages that are not always fully understood, and are difficult to model in molecular, cellular and immunological terms in mice, due to the need to introduce a very large number of human transgenes (failure to model HIV infection in mice, for example). This lecture also emphasized that there can be no infectious emergence without the capacity for human-to-human dissemination. Although this latter property is well measured by parameters such as the reproduction factor (Ro), we still know very little about the fundamental aspects of its dynamics.