Region of bacterial chromosome acquired by horizontal gene transfer and an importance contributor to the virulence of a bacterium.

Pathogenicity islands are a kind of , and obviously one that contributes to the of the bacterium within which it is found. Generally PAIs possess a number of genes that give rise to various virulence factors.

In terms of , a pathogenicity island represents a means by which relatively large numbers of can be acquired simultaneously, essentially in a pre- manner. Thus, in principle the acquisition of a pathogenicity island can result in the conversion of a bacterium into a one (that is, a pathogen).