Salmonella require multiple genes for full virulence using a complex set of interactions with its host. Many of these genes are found in 'pathogenicity islands' in the chromosome. Salmonella typhimurium possesses at least five pathogenicity islands (SPIS), which confer specific virulence traits. However, the possession of these genes does not guarantee effective infection. Tight regulation of a given virulence factor is as important as the possession of the virulence factors themselves. Thus, an understating of the regulation of virulence expression is fundamental to comprehension of any infection process and can identify potential targets for disease prevention and therapy.