1.2　Epidemiology

AIP is a relatively rare disease.There have been few epidemiological studies to estimate the prevalence and incidence of patients with AIP.The latest nationwide epidemiological survey in Japan estimated the total number of AIP patients who visited hospitals in 2011 as 5745，which meant a prevalence rate of 4.6 per 100,000 persons（Kanno et al.2015a）.Among them，newly diagnosed AIP was estimated to be 1801，corresponding to an incidence rate of 1.4 per 100,000 persons.AIP mostly occurs in the middle-aged to elderly.In this Japanese survey，the mean age of the AIP patients was 66.3 peaking at 70-79 years in men and 60-69 years in women.The male-tofemale sex ratio was 3.2.(https://www.daowen.com)