A virus transmission "risk-index" will be calculated for each group based on potential participant questionnaires enquiring, among other things, about numbers and ages of children, their possible out-of-home day care, personal properties such as smoking, chronic diseases, and potential differences in contacts during daily work. The 24 groups will be divided in groups of three most similar ones, and members each triplet then randomised in one the intervention groups or the control (see brief summary).