When cells (or sub-cellular agents such as viruses) replicate, there is a chance that the genetic material (DNA or RNA) that codes for the cell's structure and function may be changed due to the deletion, alteration or displacement of certain genes; the more cells in the population, the more likely such mutations will occur. Genetic mutation, which may also develop due to radiation, toxins or other external factors, plays a key role in the emergence of disease and has been vital to the process of evolution over the past 3.6 billion years.
Mutations of the genetic code may be favorable, benign or pathologic. Those that make the cell or organism less viable or more susceptible to environmental threats, will be deleted from the population; on the other hand, those mutations that favor survival will persist in the cell line and will be passed on to future generations.
In the case of infections, such as ebola, mutations may increase or decrease factors such as virulence. Ebola is characterized by an extremely high viral load in the tissues and fluids of the victim, increasing the number of replications and, thus, the chance for mutations to occur. While the media has reported that the ebola virus may "learn" to spread in new ways, changes in the pattern of disease merely reflect the emergence of traits (through mutation) that favor survival of the virus and are thus retained and disseminated in the viral population.
