We describe the interventions carried out in a local epidemiology unit related to the epidemiology of gastroenteritis due to norovirus in geriatric homes in Castellón (Spain) during several years. These range from various outbreaks in the same geriatric home to the study of sporadic cases which determine the endemic situation. This study focuses on the analysis of this endemic situation of digestive symptoms in silent epidemic periods and proposes a simple method to detect the initiation of GEA of progressive transmission (person-person) which we call System of Detection by Prevalence Threshold (SiDUP). The epidemiological spectrum of gastroenteritis due to norovirus in geriatric homes is discussed.