Coronaviruses frequently circulate in bat reservoirs,

occasionally spilling over into humans where most infections fail to establish

sustained transmission. A novel coronavirus spill-over event produces a virus

capable of infecting human airway cells, but initially spreads inefficiently

between people. Subsequent mutations improve human receptor binding and viral

shedding without reducing replication in animal reservoirs or humans.

Given sufficient opportunity for adaptation and global

population movement, what outcome best explains how such a virus could become

pandemic?