* Viruses are tiny infectious agents that are composed of a core of nucleic acid (either DNA or RNA) surrounded by a protein coat called a capsid. Some viruses also have an outer envelope made of lipids and proteins.

* Bacteria are single-celled organisms with a complex structure including their own DNA, RNA, and various proteins, but they are not solely composed of nucleic acids and proteins.

* Fungi are multicellular organisms with their own DNA, RNA, and complex protein structures.

* Prions are infectious agents composed only of proteins, lacking any nucleic acids.

Therefore, viruses are the only infectious agent that consists of both nucleic acids and protein.