The epidemiology and the sanitary situation of avian influenza changed dramatically with the emergence of the highly pathogenic H5N1 virus (HPAI) in 1996. As a consequence, knowledge previously accumulated on the epidemiology and the ecology of the avian influenza viruses was questioned and was required to be updated to understand the current pandemic caused by the virus (Webster, 2007; Sturm-Ramirez, 2005). This PhD combined a number of different epidemiological studies aimed at understanding the epidemiology of the H5N1 virus in the natural and human context of the Red River Delta area in Northern Vietnam. Firstly, retrospectives studies were conducted to identify the determinants of occurrence of HPAI outbreaks at 2 different scales: provincial and regional. Those 2 approaches allowed us to study the influence of the poultry production systems (provincial scale) and the influence of environmental determinants (regional scale). In addition, substantial field work was undertaken to monitor the serological and virological prevalence of HPAI in domestic poultry in our study area. After evaluation of the serological diagnostic tools being used, the data analysis contributed to a better understanding of the epidemiology of the H5N1 virus within a mass vaccination context. Furthermore, an evaluation of the vaccination strategy and implementation was also possible. In addition, to support our findings, a specific protocol to monitor the antibody kinetics of vaccinated poultry under field conditions was also conducted. Finally, a study was undertaken, in collaboration with a sociologist, to better capture the way sanitary information was circulating within our community of poultry farmers and through the formal surveillance system. Together with the results of our epidemiological work, this sociological study enabled us to propose measures to improve the surveillance and control of HPAI at the community level, to assist the people whose livelihoods were most affected.