Microorganisms are found everywhere (ubiquitous) and are essential to many of our planet's life processes. With regards to the food industry, they can cause spoilage, prevent spoilage through fermentation, or can be the cause of human illness. There are several classes of microorganisms, of which bacteria and fungi (yeasts and moulds) will be discussed in some detail. Another type of microorganisms, the bacterial viruses or bacteriophage, are examined in this study in detail.