Recurrent oral ulcer, also called recurrent aphthous ulcer (RAU), is an oral ulcerative disease with painful symptoms and periodic recurrence. Its pathogenesis is unknown, the causative mechanism is unclear, and it is the result of a combination of multiple factors, and there are significant individual differences in severity, interval and duration of the disease. Current domestic and international studies suggest that immunity, genetics and environment may be the “triad of factors” in the pathogenesis, and that the interaction of the three triggers abnormal immune responses leading to characteristic lesions. Hypothesis 1: Immune factors cause RAU RAU prodromal lesions with a large infiltration of T lymphocytes, of which T helper cells (CD4, Th) predominate in the pre-ulcer phase, T virulent cells (CD8, Ts/c) predominate in the ulcer phase, and return to T helper cells (CD4) predominance in the healing phase, suggesting that T lymphocytes play an important role in the pathogenesis of RAU …… also confirmed that the development of RAU is associated with low cellular immunity, based on the fact that the proportion of CD 3, CD 4 cells and NK cells in PBMC (peripheraI bIood monoucIear ceIIs peripheral blood mononuclear cells) of RAU patients is significantly lower than that of normal subjects. Most scholars believe that its pathogenesis is related to cellular immune dysfunction. Currently, abnormal immune function is considered to be one of the important factors in the development of RAU, and better results can be achieved if immunomodulatory drugs are used in a targeted manner. Hypothesis 2: RAU is related to H. pylori In recent years, clinical studies have found that a certain amount of H. pylori exists in the human oral cavity, and many scholars have detected H. pylori in dental plaque, saliva, and periodontal pockets, so the oral cavity may be the second important reservoir of H. pylori. From the anatomy and physiology of the oral cavity and stomach, the two are closely related, and the possibility of mutual transmission of H.pylori in the oral cavity and stomach is high, because H.pylfi in the oral cavity will enter the stomach through food and saliva, and H.pylori in the stomach will also enter the oral cavity through food reflux or fecal-oral route. H.pylori may be one of the causative agents in the pathogenesis of ROU. Studies on the correlation between H.pylori and ROU have shown that the relationship between ROU and H.pylori is very close, but there are more factors affecting the correlation study. However, whether the occurrence of recurrent oral ulcers is directly caused by H. pylori needs to be studied in depth. The correlation between oral H. pyloft and gastric H. pylori also still needs further clinical confirmation. However, as evidenced by the preliminary efficacy of the application of anti-H.pylori in the treatment of ROU, an in-depth study of the correlation between H.pylori and ROU would help to significantly improve the cure rate of ROU. Hypothesis 3: Genetic factors cause RAU Studies in monogenic inheritance, polygenic inheritance, genetic markers and genetic material have shown a genetic predisposition to the development of RAU. Studies have confirmed that the frequency of the disease is greater when it is inherited from both parents than when it is alternately inherited. In a survey of patients’ family members, it was found that the prevalence of the disease in the patient’s relatives was influenced by the level of kinship, and the closer the level of kinship, the higher the prevalence, i.e., the prevalence in first-degree relatives was greater than that in second-degree relatives and the prevalence in third-degree relatives. Meanwhile, studies on the incidence of micronuclei and chromosomal aberrations using modern cytogenetic techniques suggest that RAU patients may have certain pathological genetic qualities and abnormalities of genetic material. Hypothesis IV: Psychosocial factors cause RAU A large number of clinical practices also confirm that patients have recurrent episodes of oral ulcers in the presence of central nervous system dysfunction such as neurasthenia, insomnia, excessive tension, anxiety and irritability. After the above-mentioned symptoms are improved or cured, the oral ulcers improve significantly or heal spontaneously, the interval between attacks is prolonged, and the symptoms and signs are reduced or not obvious, thus indicating that RAU is closely related to autonomic dysfunction. Therefore, psychotherapy such as sedation and relaxation or the administration of anti-anxiety and anti-depressant drugs have been clinically used and certain therapeutic effects have been received. To investigate the pathogenic mechanism, current research suggests that psychological behavior changes such as trauma, anxiety and depression lead to changes in hormone levels and temporary immunosuppression, which revives the virus that has been integrated and latent in the host cells, and the viral antigen is expressed on the surface of epithelial cells, resulting in oral ulcers due to the host’s immune response to the viral antigen. Finally, the causes behind a small and common oral ulcer may be very complex. Now that medical research has entered the genetic era, the pathogenesis of RAU will be fully revealed in the future, laying the scientific foundation for the treatment of the disease. It is believed that with further in-depth research, people will uncover the pathogenesis of RAU and find effective ways to prevent and treat the disease.