Despite the availability of numerous drugs and the impact of their use, treatments continue to fail, often due to the development of resistance.  During drug therapy, low-level virus replication still occurs, particularly when a patient misses a dose.  During replication the virus makes errors in copying its genetic material and if a mutation makes the virus resistant to one or more of the drugs it may begin to replicate more successfully in the presence of that drug and undermine the effect of the treatment. If this happens then the treatment needs to be changed to re-establish control over the virus.