* Viruses: Viruses are microscopic infectious agents that can cause disease. They are considered biological agents.

* Toxins: Toxins are poisonous substances produced by living organisms, including bacteria, plants, and animals. These are biological in origin and can be used as biological weapons.

* Bacteria: Bacteria are single-celled organisms that can cause a wide range of diseases. They are definitely biological agents.

* Blood: While blood contains biological components like cells and proteins, it's not a biological agent in itself. Blood is a bodily fluid, not an organism or a toxin that causes disease.

Therefore, blood is the only option that doesn't directly fit the definition of a biological agent.