The norovirus outbreak occurred during an evening dinner at a hotel restaurant , as shown in Figure 1. The index patient was a woman who vomited onto the floor at 8:30 p.m. The vomiting was not projectile, and the vomitus was rapidly cleaned by a waiter. Since handwashing of the lady and the waiter might not be effective enough , we assumed that very small amounts of norovirus were initially carried by the index patient’s and the waiter’s hands at the beginning of the outbreak (Table S5). Because the meal was not disturbed by the vomiting episode, the following 90 min were assumed to be the potential exposure period.