Biological agents include all of the following, except:

Biological agents are microorganisms or toxins that can cause illness or death in humans, animals, or plants. Common examples of biological agents include viruses, bacteria, and biological toxins. Options such as Ebola, Anthrax, and Smallpox are all classified as biological agents.

Ebola is a viral hemorrhagic fever caused by the Ebola virus, Anthrax is caused by the bacterium Bacillus anthracis, and Smallpox is a highly contagious disease caused by the variola virus. All of these are significant concerns in bioterrorism and public health.

Phosgene, on the other hand, is a chemical compound that is classified as a chemical agent, not a biological agent. It is a colorless gas that can cause severe respiratory damage and was used as a chemical weapon during World War I. Its classification as a chemical agent distinctly separates it from the biological agents listed, making it the correct answer for the question.