Some mutations may be deleterious to the virus as a whole, and the new mutant may quickly 'die off'. However, simple mutations resulting in changes in the sequence of amino acids in a protein, called 'substitutions', may be retained. Two to three stable amino acid substitutions may arise each year in influenza. Should these 'drifts' occur in the HA or NA proteins, the antigenic face of the virus, they may produce a subtly different immunogenic profile in a new round of human infection. These types of genetic changes are not particularly pronounced, and although 'drifted' strains of influenza can cause localised outbreaks, they may also produce large epidemics.
Antigenic drift can occur in both A and B influenza viruses.
The segmented virus genome of influenza makes possible the interchange of genes from different viruses. This re-assortment can occur when two different influenza A, but not B or C, viruses are present in the same cell. Importantly, influenza A can infect a large number of animal types in addition to humans. In terms of strains dangerous to people, this phenomenon is believed to occur principally in the pig - an animal than can be infected by both bird and human strains of influenza. The new, re-assorted form will have a distinct selective advantage in human infections since the antibodies to HA and NA will not recognise the 'newcomer'.