Answer
HIV mutates rapidly and attacks the cells designed to defend against it, frustrating attempts to make a lasting vaccine. In the former case, it resembles influenza, but not smallpox which is rather stable genetically.
Work Step by Step
Consider the main notable features of HIV--its mutability and its assaults on the immune system. Then consider how this affects the ability of the immune system to attack it, now and over time. Then, look in the chapter for any similar viruses, and also compare these features to those of smallpox.
