The influenza virus is particularly prone to creating mutations during cell replication. That is to say that the influenza virus frequently makes “copies” of itself that are different from the parent virus. When the difference is minimal, the host animal will have some immunity, so infection will cause only minor symptoms — something like a cold. But when the mutation results in a version that is drastically different from the parent, the host may have very little or no immunity, and the result is a pandemic.
Influenza occurs in a number of species — humans, pigs, and birds, for example. Usually influenza is passed on within a species, humans do not infect humans with bird flu. But humans may catch avian flu from birds — should they, for example, go to a poultry farm where there is influenza in the bird population. When one species in infected with influenza from another, there is the possibility for intermingling of the elements of virus from the two species — this is called reassortment.[1]
This ability of influenza viruses to re-invent themselves with great speed drives researchers to be diligent and vigilant in their efforts to be prepared for another pandemic. These efforts resulted in the creation of ten different vaccines compositions between 1986 and 1998. [2] ↩
1. Alan J. Hay, Victoria Gregory, Alan R. Douglas and Yi Pu Lin, “The Evolution of Human Influenza Viruses”, Philosophical Transactions: Biological Sciences, Vol. 356, No. 1416, The Origin and Control of Pandemic Influenza (Dec. 29, 2001), pp. 1861-1870, passim, <http://www.jstor.org/stable/3067061> accessed on 20 March 2014.
2. Hay, et al., “Evolution …”, p. 1863.