this is the host shift of the influenza A H1N1...if you can see, there are a red line is a connection between human and pig, so you can think for yourselves about that. take a look at the antigenic shift, and then you will understand the virus better...this make no sense, but it will make a big change...
picture taken from wikipedia
for more information, go to
http://en.wikipedia.org/wiki/Influenza_A_virus_subtype_H1N1
