Question: The virus that causes swine flu, H1N1, originally could only infect pigs. People who raised pigs …

The virus that causes swine flu, H1N1, originally could only
infect pigs. People who raised pigs were exposed to this virus all
the time, but the host range of the virus was restricted to just
pigs. Then all of a sudden, H1N1 gained the ability to infect
humans and the swine flu pandemic began. As a geneticist, how would
you explain this? You may answer in very general terms. A full
credit answer can be written in one word