Retrovirus

A retrovirus is any virus which copies itself as part of the cell's DNA by reverse transcribing its RNA. Since the cell cannot proofread the step in which RNA is converted back to DNA, errors often go unnoticed. This changes the exact sequence of a retrovirus' RNA frequently. This makes it harder for retroviruses to be attacked by drugs.
HIV is a retrovirus, which makes it very difficult to overcome using drugs.