The Authors present an exhaustive review on microbial agents of appendicitis by means of literature and personal research data. Thus, a detailed analysis is made on common autochthonous agents and their pathogenetic interactions and on less common exogenous bacterial, viral, mycotic, protozoan and helminthic agents with emphasis to the role of Yersinia enterocolitica. In fact this bacterium seems responsible for 3% to 8% of cases in accordance with literature and personal research data (more detailed, Y. enterocolitica has been isolated in 3.8% of 208 inflamed appendices from both pediatric and adults surgical florentine patients). At the end, the pathogenetic role of "new" other bacteria, like Buttiauxella agrestis, Aeromonas hydrophila, Arizona, Streptococcus lactis, is debated on the basis of a personal study.