Statistics. There have been rare cases of the incubation period being longer than 21 days, but most people will get sick much sooner. (95th percentile) That sa

Statistics. There have been rare cases of the incubation period being longer than 21 days, but most people will get sick much sooner. (95th percentile) That said, there was a recent PLOS article that suggested that a longer waiting period might be called for: http://currents.plos.org/outbreaks/article/on-the-quarantine-period-for-ebola-virus/ http://currents.plos.org/outbreaks/article/on-the-quarantine... There's also a cost-benefit analysis involved here. More transmissible/serious diseases call for a larger margin of safety, while at the same time enforcing a longer quarantine takes a toll on total available infrastructure.