The length of time hunter-gatherers stayed in one region depended on climate conditions other nomadic people available food amount of stored food soil fertility

The length of this time depended on many factors but from the ones you've listed here, avaliable food amount was surely the most important one. When there wasn't any food left in a certain environment, this meant that there was no reason for a nomadic hunter-gatherer people to stay in that place anymore. 

An epidemic is a large and localized outbreak of a sickness or disease.  a pandemic is a large global outbreak of a sickness or disease.