Microbiology is the study of microorganisms - little, really little, critters (except for The Big One). And, please take a look at some relative sizes of different living cells at Jim Sullivan's Page: Cells Alive! These "bugs" include: bacteria (that's the Latin plural for bacterium); viruses (that's the non-Latin plural for virus - virii sounds weird, so I don't say it); and, fungi (that's the Latin plural for fungus - which by now you have guessed, or already knew, and may not be all that interested to know, anyway). Microbiology is actually made up of several sub-disciplines which individually may stand alone, because there is so much to learn in each. These disciplines include: Immunology (the study of the immune system and how it works to protect us from harmful organisms and harmful substances produced by them - is what I, Marci, and Larry work on; Virology, the study of viruses, and how they function inside cells - Marci does some of this, too; Pathogenic Microbiology, the study of disease-causing critters and the disease process - is what Eric does; Microbial Genetics, the study of gene function, expression, and regulation - is what Susan and Del do - although Del mostly examines mutations in genes and substances which appear to prevent mutations; Physiology, the study of biochemical mechanisms - is what Jim and Clarence do. I'll focus on bacteria right now, not because this group of critters is necessarily more interesting, but because I know very little about fungi, and because I don't want to talk about viruses just now.
Bacteria are absolutely necessary for all life on this planet - for every known ecosystem - including the human ecosystem! Without bacteria, there would be no life, as we call life, on the earth. However, it is a good thing that most bacteria die-out. Here is why: bacteria are single-cell organisms, that produce more of their kind by cell-division, alone. So, if one begins with a single bacterial cell like E. coli for example, in 20 minutes there will be two, and 20 minutes later, four, etc., E. coli cells. At this rate, even though most bacteria are several hundred-times smaller than we can see with our naked eye (never seen a clothed eye), in only 43 hours, from that one cell at the beginning, there would be enough E. coli to occupy the entire volume of the earth (1,090,000,000,000,000,000,000 cubic meters)! In only about two additional hours, these bacteria would weigh as much as the earth - 6,600,000,000,000,000,000,000 tons! Bummer! Luckily for us, most bacterial cells die because of the enormous competition for food, and because of other tiny organisms which produce substances (antibiotics) that kill them - you know, like penicillin, which is made by a particular fungus, the mold - Penicillium). Thank goodness for that one, huh? Actually, many antibiotics are made by certain bacteria too, and, we get many of our necessary vitamins and nutrients from bacteria by allowing the bacteria to multiply in number, and isolating the things that they make, that we cannot make. For example, amino acid supplements are available ("enriched" bread simply means that the amino acid, lysine, which we absolutely need, but cannot make ourselves, is added to the flour used to make the bread), to provide one additional source which most people will eat. This amino acid is produced by certain bacteria grown in huge vats (can be 20,000 liters at one time - that's about 1,500 gallons!), and purified for our use. Antibiotic production is similarly done.
With the advent of molecular genetics and recombinant DNA technology,
bacteria now play a very important role as producers of human
substances. Since we have learned how genes function, we are able to
introduce a human gene into a bacterium and have the product of the human
gene expressed. Consequently, a hormone called erythropoietin, which is
absloutely necessary for the proper development of red blood cells
(erythrocytes), but very, very, difficult to isolate, is now available in
high quantity. People who do not have kidneys cannot make this hormone;
however, because the hormone has been cloned into bacteria, plenty of
this hormone can be made, purified, and given to these people. Human
insulin can be similarly made. These are only two examples of the many
substances now available to treat human disorders because of our
understanding of bacteria.