Pathogenicity island

Pathogenicity islands (PAIs), as termed in 1990, are a distinct class of genomic islands acquired by microorganisms through horizontal gene transfer. Pathogenicity islands are found in both animal and plant pathogens. Additionally, PAIs are found in both gram-positive and gram-negative bacteria. They are transferred through horizontal gene transfer events such as transfer by a plasmid, phage, or conjugative transposon. Therefore, PAIs contribute to microorganisms' ability to evolve.

One species of bacteria may have more than one PAI. For example, Salmonella has at least five.

An analogous genomic structure in rhizobia is termed a symbiosis island.