The thesis discusses the pathogenesis, nature and treatments of primary endodontic infections. The main focus is the microbial aspect of root canal infection. Different bacterial localisation in different stages of root canal infection are elaborated. Virulence factors in endodontic infection are also explained, shining light on the pathomechanism of root canal infection and the relationship with their clinical presentations. Other clinical traits are also mentioned such as clinical aspects, symptoms, diagnosis and treatment possibility.