Viruses are the smallest of the infectious disease causing agents. They contain either RNA or DNA (but not both) within a protein coat or virion. The viral nucleic acid encodes the make up of this virion as well as some enzymes required for viral replication.
The proteins making up the virion are antigenic. This means they are able to induce an immune response in persons infected with the virus. Antibodies produced by an immune response are specific to that particular virus.
Small changes in the order of the bases in the nucleic acid cause slight changes to the amino acids incorporated into the virion. While not affecting the major structure or infectivity of the virus these small changes do change the antigens present in the virion. In each generation of virus particles the number of changes to the antigenic structure increases.. This gradual change over time is known as either antigenic drift or genetic drift.
Eventually these changes are sufficient for the virus to infect someone even if a previous infection left them immune to an earlier antigenic type. When sufficient antigenic drift occurs within a virus, that the majority of a population is non-immune, there is a danger of a viral epidemic
Within cells, DNA acts as a template for the replication of new nucleic acid strands. Both RNA and DNA require a DNA parent strand. Normally RNA will not provide such a template. The replication of DNA is subject to a number of control processes to ensure that the DNA or RNA daughter strand produced contains no errors.
Viruses use the enzymes within the cells, which they infect, to help produce or replicate new viral particles. RNA viruses contain an enzyme that allows RNA to replicate RNA but there are no control measures available within the infected cell to ensure correct replication of the new RNA strand. Without such checks on the replication of their nucleic acid, viruses containing RNA are particularly prone to genetic drift.
The most common of the RNA viruses to exhibit genetic drift are the flu viruses, Influenza A and Influenza B. Each “flu season” the slight change in the antigenic structure of the circulating viruses requires the production of a new vaccine formulation to maintain immunity in vulnerable patient groups.
While responsible for some epidemics of influenza genetic drift does not lead to pandemics such as the Spanish Flu of 1918 or the more recent swine flu pandemic. Antigenic shift causes of such major pandemics.