Horizontal gene transfer (HGT) plays a key role in the evolution of bacterial pathogens. The exchange of genetic material supplies prokaryotes with several fitness traits enhancing their adaptive response to environmental changes. Pathogenicity islands (PAIs) represent an important and in most cases already immobilized subset of the different vehicles for HGT. Encoding several virulence factors PAIs represent a major contribution to bacterial pathogenicity. Nonetheless, the transfer mechanisms of PAIs still remain elusive. We summarise the currently available data regarding the major ways of genetic mobilisation with a focus on the transfer of the
High-Pathogenicity Island (HPI).