Which of the following is a type of biological agent?

A virus is classified as a biological agent because it consists of living organisms that can cause disease in humans, animals, or plants. Biological agents include a wide variety of microorganisms, such as bacteria, viruses, fungi, and parasites. These agents are capable of reproduction and can replicate within host organisms, leading to infection and disease.

In contrast, nerve agents, choking agents, and blister agents are all chemical agents. They are classified under chemical warfare and are designed to induce harm through toxic effects rather than through biological processes. Thus, they do not fall under the category of biological agents, which is why the selection of a virus as the correct answer highlights its role as a living organism that can cause infections.