OVALAU is tidally locked with its primary, with one hemisphere always facing the star.  Its ecosphere hence consists of a narrow band between the too-hot sunny side and the too-cold dark side.  Overlauers are very resourceful in dealing with this constant climatological balancing act, but a history of dangerous epidemics has left scars on their collective psyche.  Whenever an outbreak (or even a rumor of one) occurs, everyone hurries away from the presumed-infected zone, clogging the congested paths along Ovalau's prime meridian.  Some even opt for a life-threatening journey across the uninhabited wastes, desperately seeking the antipodes of disease.  After an awkward interval waiting for the plague to dissipate, they all eventually go back home to what's left of the rest of their lives.

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What about the denialists? Well there were some, but the gene has been weeded out as they keep denying the dangers of the bright side.