In a sense, HVAC engineers are the physicians of the built environment because IAQ and water management determine the growth and transmission of microorganisms. Therefore, it is essential for engineers to understand the basic characteristics of these minuscule beings.
Many people think of “germs” as one big group of tiny (how tiny?) organisms that cause infections. There are, however, several different categories of microorganisms, and within these categories only some cause infections or disease. These are often called pathogens.