Microbiology was the branch of biology that dealt with the detection and identification of microscopic forms of life, such as various microorganisms and viruses. Scientists who practiced in this field were known as microbiologists. The area of microbiology that focuses on bacteria and their relations to medicine, industry, and agriculture was known as bacteriology.
During the 1990s, Earth was in the process of recovering from bacteriological warfare experiments. As a result, portions of the population became infected with a virus that resembled the one that ravaged Omega IV during the 13th century, well into the 23rd century. In the case of the Omega IV virus, according to Doctor Leonard McCoy, "as the years passed, nature built up natural immunizing agents in the food, water, soil." As a result, the inhabitants of the planet we extremely long-lived. (TOS: "The Omega Glory")
When B'Elanna Torres discovered a piece of Srivani sub-molecular technology on a hyper-stimulated segment of Chakotay's DNA, she told The Doctor, "I'm no microbiologist, but that doesn't look like it belongs there." (VOY: "Scientific Method")