240 Jean-Paul Gonzalez et al. of view in the search for a cause. Studies of disease dynamics in vertebrate populations must take into con- sideration interactions with other components of the bio- cenosis and, more generally, the ecosystem (Pontier et al., 2009). Likewise, it is essential to consider diseases as being potentially dependent on each other and to take into ac- count environmental factors capable of modulating disease dynamics. The concept of pathocenosis is particularly useful for understanding the current emergence of infec- tious diseases, as the underlying mechanisms include interactions of infectious agents both with one another and with the environment. Given the complexity and diversity of possible situa- tions, one must ﬁrst identify the key elements needed to provide a framework for studying pathocenosis and its dynamics, and eventually predict and prevent disease emergence. This framework could include the studies of: 1) the mechanisms of disease emergence within an established pathocenosis; 2) a typology of conditions leading to disease emergence; and 3) the spatial and temporal scales at which the pathocenosis can be understood. Grmek (1969) already noted that detecting transitions from endemic to epidemic states faces major but surmountable difﬁculties, and re- quires accurate diagnosis of all sick individuals in a pop- ulation during a given period. The obstacles vary with historical times, but they must be overcome and, in a contemporary analysis, this requisite can be largely fulﬁlled. The identiﬁed general mechanisms then can be used to deﬁne the conditions of the greatest disease emergence risk (Pontier et al., 2009). The main challenge in understanding the succession of pathocenoses is ﬁnally to determine the spatial scale at which the phenomena must be studied (e.g., at the level of the village or the continent). Two typical examples are the hemorrhagic fevers of South America, caused by arenaviruses, which cocirculate within their natural hosts (rodents), and seasonal inﬂuenza pandemic. Rodents are found throughout the continent, but the are- navirus remains conﬁned to areas of a few hundred square kilometers, and outbreak studies will generally focus on individual villages. In contrast, the prevention of seasonal ﬂu pandemics requires, obviously, a global approach. While most examples and references given in this article refer to infectious diseases (due to viruses, bacteria, parasites, and fungi), the pathocenosis also encompasses chronic diseases (diabetes, cancer, allergies), degenerative diseases (systemic diseases, cardiovascular diseases), and other health disorders (mental illness, trauma). The hygiene hypothesis is based on a similar approach, directly linking the increased frequency of atopy and allergy to the reduction in the incidence of several infections, especially during childhood (Bach, 2005). The pathocenosis, as an integrative concept, helps us to consider health status holistically and in all its complexity, where the monodis- ciplinary approach prevents us from grasping the full complexity of a phenomenon. Moreover, the clinical expression of a given infection can vary with the charac- teristics of both the pathogen (genetically dependant viru- lence and infectivity) and the host (age, gender, genotype, physiological state, other infections). A better understand- ing of the dynamics of infections and the importance of asymptomatic infections thus requires all this information to be taken into account. CONCLUSION The principal lesson of the pathocenotic concept is that disease prevention requires a global, comprehensive, and integrated approach. This important idea has been left behind during almost 40 years since Grmek raised it in 1969, likely due to the lack of data and methods required to test it. However, the recent methodological advances in ﬁeld and theoretical work have recently enabled us to highlight many solid examples of diseases interactions. These studies need to be continued, since they are the only way to evaluate the importance of infections’ interactions in epidemic patterns and disease emergence, and thus conﬁrm that pathocenosis is more than an elegant idea proposed by a philosopher of science. This question is of major importance because, in the latter case, this concept should permit a revisiting of our knowledge about infec- tious diseases in an original manner, for better compre- hension and more efﬁcient control and prevention. ACKNOWLEDGMENTS Financial support has been provided by the French National Agency for Research (ANR SEST ‘‘Pathoce´noses et e´mergence des maladies transmissibles: un concept uniﬁ- cateur mis a` l’e´preuve sur des pathologies exemplaires’’). We thank our colleagues from the ANR Pathocenosis Team for productive discussions, namely Philippe Barbazan, Aﬁf Bensalah, Koussay Dellagi, David Fouchet, Ste´phane Marchandeau, and Marc Souris.