The group of pathogens that are one-celled microscopic organisms are bacteria.

Here's why:

* Bacteria are single-celled prokaryotes, meaning they lack a nucleus and other membrane-bound organelles.

* Viruses, while microscopic, are not considered living organisms because they cannot replicate on their own. They need a host cell to reproduce.

* Fungi can be single-celled (yeasts) or multicellular (molds and mushrooms).

* Parasites are multicellular organisms that live on or in a host and obtain nutrients from their host.

Let me know if you have any other questions!