I study bacteria and I am most interested in the bacterium called Staphylococcus aureus . The next group of organisms we study are the fungi. They include mushrooms, molds, and yeasts. However, microbiologists usually only study the molds and yeasts. We leave the larger fungi to the biologists.
Yeasts are still single celled organisms like the bacteria.
The yeast cells are usually about 5-10 times bigger than bacteria but you still require a microscope to see them. In fact an antibiotic called penicillin is obtained from one of those nasty old bread molds. To see any details however still requires a microscope. Examples of the molds and yeast can be seen at Mycology Online . The last group of organisms studied are the parasites.