Why are Influenza viruses changing all the time?
Influenza viruses have the peculiarity of possessing a genome made up of 8 distinct gene segments (coding for 11 viral proteins). If pigs or other animals (and also humans) become co-infected with two different strains of influenza viruses this may cause a partial mixing of the genetic material and a third, new virus may be generated with genes coming from both the original viruses. This mixing (called re-assortment) is largely due to the segmented nature of the genome. Reassortment may or may not change the antigenic characteristics. These reassortments may also bring changes to other virus characteristics, including virulence and host range. Re-assortments occur frequently, including when viruses from different animal species infect the same host. Swine influenza viruses do not infect humans frequently. When they do, such infections in humans are mostly associated with direct and close contacts with infected pigs. Human-to human transmission is usually not observed.
