Microbiology refers to the study of microorganisms. As the name suggests, microorganisms are organisms that are so small they can only be seen using a microscope.
Bacteria, fungi (such as yeasts and molds), protozoa, viruses, algae, and some parasites are all types of microorganisms. Some people also list prions as microorganisms even though they are proteins and not actually organisms.
Unfortunately, microorganisms also can be detrimental. They are the cause of many diseases in humans, animals, and plants.
Disease-causing microorganisms are called pathogens. A pathogen, or the substances it produces, must invade the human, animal, or plant body to cause illness.
While many diseases are transmissible from person to person or from animals to humans, only a few are transmitted through foods.
Diseases that are caused by eating food are usually referred to as food poisoning or foodborne illnesses. This guidebook serves as a broad and very basic introduction to the microorganisms associated with food production, and in particular, bacterial pathogens.
Source:
https://www.fsis.usda.gov/