There appear to be limits on how advantageous a high mutation rate can be, and there is evidence that mutation rates (as determined by polymerase fidelity) are under selection to be neither too high, nor too low. An exciting extension of the idea that mutation rates can be too high is that drugs can be used to increase the mutation rates of pathogens to intolerable levels. Studies have shown that treating RNA viruses such as poliovirus with ribavirin produce results consistent with the idea that the viruses mutated too frequently to maintain the integrity of the information in their genomes[2]. This is termed error catastrophe.