Read an introduction to viral evolution and you'll pretty quickly come across a sentence equating to 'viruses mutate and evolve fast and this is because their polymerases lack proofreading ability'. Sure, but that's a big generalisation. Large DNA viruses, for example, tend to have polymerases that are much less error-prone (again a generalisation). Nevertheless, in the world of RNA viruses it is a good rule of thumb that the polymerases tend to be error-prone when copying their genetic code.
Rozen-Gagnon, K., Stapleford, K.A., Mongelli, V., Blanc, H., Failloux, A-B., Saleh, M-C., Vignuzzi, M. (2014). Alphavirus Mutator Variants Present Host-Specific Defects and Attenuation in Mammalian and Insect Models, PLOS Pathogens, DOI: 10.1371/journal.ppat.1003877