The influenza virus is well-known for its fast mutation rates, which frustrate scientists who develop vaccines and treatments. While these viruses have fast mutation rates, one of the most rapid and dangerous transformations happen in the form of something called antigenic shift. Antigenic shift occurs when two strains of a virus, usually influenza but it could be any viral pathogen, combine to form a new mutant subtype that includes a genomic reassortment.
Antigenic shift became a great concern as fears about avian flu began to spread a few years ago. Imagine a situation where a virus that infects birds manages to jump hosts and infect a pig. At the same time, a human flu virus jumps to the same pig. While these two drastically different forms of the flu are infected the same cells, the DNA is mixed in the hosts and released as a new hybrid virus strain. This new strain would have surface antigens from both the human virus and the flu virus, potentially allowing it to jump to humans with the viral features of the bird flu. Without any resistance to any form of bird flu, humans would be very susceptible to this new strain and it could pose a tremendous threat.
Antigenic shift is generally considered to be synonymous with any genetic mutation that results in the ability of a strain to jump from one species to another. In that definition, a hybrid strain doesn't have to occur. Instead, if the bird flu mutated and could jump directly to a human host, this would be an example of antigenic shift.
It is generally accepted that birds are the original source of all influenza A type viruses. These viruses are prevalent in bird populations and some kind of antigenic drift results in their transmission to other organisms.
This kind of genetic mutation is directly contrasted with antigenic drift. Antigenic drift is the more common natural mutations of influenza or other viruses. These kinds of mutations are a result of unrepaired errors in the genome that result in new strains that can evade resistance, but don't necessarily make the virus able to jump species.
Another virus known to have mutated heavily by antigenic shift is HIV. Superinfection, or the process when two different viruses infect the same organisms and as a result form a new hybrid strain, is generally considered to be the mechanism for HIV transmission from apes to humans. Multiple slightly different HIV strains may have infected the same organism, recombined their genetic elements freely, then produced a new reorganized HIV virus.