Reassortment
Reassortment can occur when one animal is infected by two different strains of influenza virus. If these two different strains mix in a cell, they can exchange one or more of their eight genetic segments resulting in a virus that may have characteristics from each of the "parental" strains. This may result in a virus with properties that were not observed in either donor strain. Two pandemics in the 20th century were thought to be caused by this mechanism. These pandemics are said to be due to antigenic shift. The change in the viral genome happens very quickly and results in a virus with a new combination of genes.
In this example, a host is co-infected with Parental strain A and Parental strain B. The circles represent different Partental flu viruses. Each cylinder represents a different genetic segment within the virus. A new Reassortant strain results when the HA gene from Parental strain A (yellow) is replaced with the HA gene from Parental strain B (blue).
Blogs
The World Killer - The reassortment risk is real
February 22, 2010
NS from H5N1 + H7N1 = Trouble
February 28, 2010
Pandemic flu hybrid virus in Chinese pigs
March 2, 2010
Influenza hybrids made to order can be more infectious than their parents
March 8, 2010