Viruses are a type of biological agent that needs a host to replicate and spread. An example of this biological agent is:

Viruses are indeed a unique class of biological agents that require a living host for their replication and propagation. In the context of the question, smallpox is correctly identified as a viral agent. Smallpox is caused by the variola virus, which infects humans and relies entirely on human hosts for transmission and survival.

In contrast, HIV, while also a virus and therefore similar in its reliance on a host, is part of a different category that focuses specifically on the impact it has on the immune system. Anthrax and botulinum toxin represent different types of biological threats: anthrax is caused by a bacterium (Bacillus anthracis) that can reproduce independently of hosts and can form spores for environmental survival, while botulinum toxin, produced by the bacterium Clostridium botulinum, is a potent toxin rather than a living organism itself. Thus, smallpox stands out as the quintessential example of a viral pathogen that meets the criteria stated in the question.