If the modified virus is lucky enough to encounter the new

can occur. We call this a spillover infection. The virus has spill over
into a new type of host. Early on, during the spillover event, the new virus is not very
at infecting the new host. Because of that, many spillover viruses
species,
are extinct after infecting one or two people. However, if the virus survives and
reproduces long enough, natural selection will promote any new
that help it spread and
in the new host population.