Campylobacter jejuni is a microaerophilic and thermophilic pathogen genetically close to Helicobacter pylori. It represents one of the most of concern emerging food pathogens, as it is one of the primary sources of human enteritis, in developed as well as in developing countries. It is also a source of rare but serious neuropathies. Interest in this bacterium has constantly been growing during the last decade, since many antibiotic resistant strains are emerging. The sequencing of C. jejuni genome has revealed a small genome in which a lot of regulation systems are fully understood in well-studied bacteria were absent. To infect humans, a pathogen has to survive different environmental stresses and attacks of the immune system, and also be able to express its virulence factors. The Campylobacter conundrum is the latter: how does this microaerophilic (=sensitive to oxygen) bacterium, owning a small genome and lacking most of the well-known regulation systems, still constitute a hazard for humans? From what is known about C. jejuni genome, this book proposes to draw a parallel between on the one hand the different mechanisms and regulation pathways existing, and on the other hand stress resistance and virulence of this emerging food pathogen.