Helicobacter Pylori (H. pylori) is a gram-negative bacteria infecting numerous 
people all over the world. It has been established that H. pylori play an 
important role in pathogenesis of gastritis, peptic ulcer and gastric cancer. 
Pathogenic features of this bacterium are mainly attributes to the existence of 
pathogenic islands (PAI) genes. The most known genes in these islands are 
cytotoxin-associated gene A (CagA) and vacuolating cytotoxin gene (VacA). Most 
studies demonstrated various frequency of CagA and VacA in patient with peptic 
ulcer or gastritis in different countries. This variation in CagA and VacA 
frequency may be due to the capability of this bacterium to be genetically 
versatile and can alter the expression of these genes with geographic diversity. 
Although H. pylori infection is not usually associated with any clinical 
symptoms, but sometimes leads to inflammation in gastrointestinal system and 
resulted in peptic ulcer and gastric cancer. In this regard, this review will 
illustrate the importance of Helicobacter pylori in pathogenesis of 
gastrointestinal disorders with focusing on CagA and VacA virulence factors.