Devil&#x27;s advocate: I think the argument is that the damage of the disease is disproportional to the people exposed, because the increased <i>pairs</i> of interactions are what can make it spread fast.<p>So doubling the number of people congregated in Rio won&#x27;t just double the danger, but quadruple it.  If car accidents obeyed a similar dynamic -- where they increase (much) more than proportionally to the number of vehicle-miles driven, we should be similarly more cautious about additional drivers.