Salmonella is a Gram negative bacillus that behaves like a facultative intracellular pathogen. Its environment is the human and animal gastrointestinal tracts, it is never found like a normal microbiota. It is associated with gastrointestinal problems, septicaemic disease and abortion, due to its cellular invasion capacity and its intraphagocytic survival. Nowadays, it is known that Salmonella contains five pathogenicity islands. Several genes involved in the cellular invasion of nonphagocytic cells such as epithelial cells, apoptosis of macrophages, activation of routes of MAP kinases and transcription factors are located in centisome 63, constituting the pathogenicity island 1 (SPI-1). The SPI-2 and SPI-3 islands control the intracellular survival and replication. The SPI-4 island encodes a putative type I secretion system and its believed that it participates in the intracellular survival. Finally, the SPI-5 island encodes for factors involved in the fluid secretion and inflammatory reaction in the intestinal mucosa. Due to a coordinated and precise regulation of the Salmonella genes, it allows for adaptation to environmental changes that occur during an inflammatory process.