saliva + hot liquid are pretty good disinfectants, especially if only dry lips are used..

related side note: most (all?) eastern christian groups use a single spoon for communion..

my unscientific biological theory on these things is they to some extent cause immune-boosting results since you are getting a micro exposure to mostly killed off or weakened pathogens in combination with 'clean' nutrients (mate+vitamins or host) and potentially also antibodies from the others in the group..