The primary objective is to measure the decrease of the number of secondary cases of flu in households where the index case will wear a surgical mask during its period of infectiousness (5 days), compared to households where the index case will not be subject to this intervention. Secondary objectives are: (1) to study the feasibility and tolerance of wearing a mask continuously during the period of viral shedding; (2) virological description of infection spreading among household contacts based on virological collections.