no viral infection happens in a vaccuum. Each year's seasonal and nonseasonal flu comes on the backs of years of "less people" for it to kill, and people who h

no viral infection happens in a vaccuum. Each year's seasonal and nonseasonal flu comes on the backs of years of "less people" for it to kill, and people who have gained immune responses to any of its closest relatives, and people who have been immunized, and even humans who have been culled by similar viruses milennia ago during migratory bottlenecks.