The present application relates to infectious diseases, pathogenic organisms or pathogenic antigens, and the immune responses that are the body's first line of defense thereto. The application enables medical protocols and products inter alia for treating or preventing or limiting the dissemination of an infectious disease. Methods and compositions are disclosed which employ dIgA for assessing functional immune responses to a pathogen, and in prophylactic or therapeutic compositions. In particular embodiments, the methods and compositions enhance the armamentarium for those charged with managing infectious diseases and populations exposed to highly transmissible and potentially debilitating or fatal pathogens such as those causing epidemics. One particular infectious disease is COVID-19 caused by the virus SARS-COV-2.