"Because our target audience is the students, the editorial committee felt that some information in the previous edition was missing in order to the newcomer to better understand the world of poultry medicine. Commercial poultry production is rapidly evolving, not only new pathogens regularly emerge, but given the trend to reduce and eliminate the use of antibiotics, ‘old’ diseases are resurfacing. Furthermore, recent access to relatively cheap and rapid genome sequencing techniques is changing the face of pathology. Determination of genetic relatedness among bacteria has clarified pathogen classification while bringing new taxonomy. Furthermore, sequencing of the complete genomes of several pathogens, and comparative genomic analyses of commensal and virulent strains have facilitated the identification of a number of virulence-associated genes and is helping scientists to further understand pathogenesis and to develop new preventive tools, hence the need for re-edition of this manual."