Influenza type A (Flu A) viruses are especially prone to reassortment due to their wide host range (humans, dogs, birds, pigs, horses, whales, seals, and other animals). Specifically, the Flu A genome is made up of eight loosely linked segments, each of which harbors at least one important gene. Those genes direct the expression of the major viral proteins such as hemagglutinin (HA) and neuraminidase (NA). In the process of viral reproduction, the linkages between the eight segments of the Flu A genome break apart. Since it is possible for two different Flu A strains to infect a cell simultaneously, some of the genetic segments from one strain can be swapped with another during reproduction. For instance, if a human flu virus and a bird flu virus infect a person, reassortment can intermingle genes from both viruses during replication and create a virus with a protein against which humans have little or no immunity, plus human influenza genes that are more likely to cause sustained human-to-human transmission. In contrast, Influenza B (Flu B) and C viruses do not cause pandemics, most likely due to their limited host range of only humans.
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