Here's a breakdown:
* Viruses are tiny infectious agents that require a host cell to reproduce. They are much smaller than bacteria and have a different structure.
* Fungi are eukaryotic organisms that can be single-celled or multicellular. They obtain nutrients from their environment, often by decomposing organic matter.
* Bacteria are single-celled prokaryotes that can cause a variety of diseases. *Yersinia pestis*, the bacterium responsible for the plague, is a highly infectious and deadly pathogen.
So, while all three can cause diseases, the plague specifically is caused by a bacterium, not a virus or a fungi.