Viruses that cause chronic infections, such as HIV and hepatitis C, are genetically highly variable due to a high rate of error during replication. This high rate of mutation allows for the evolution of multiple strains within a host and leads to competition between strains, with only the fittest surviving. I will introduce the within-host model, which incorporates viral mutation, and briefly discuss how we are linking this to a between-host model in an effort to examine viral fitness.