The importance of microbiology in the design of heating, ventilating, and air-conditioning systems is only tangentially considered by architects, air-conditioning engineers, and systems engineers. Airborne disease remains but a theory of microbiologist; indeed, most physicians and epidermiologists ignore it, preferring to deal with readily demonstrable vectors—fingers, food, water, inseacts, and like fomites. This chapter provides selected examples of environments that foster sickness among occupants which should result in a compelling consensus of support for multidisciplinary action aimed at healthy as well as comfortable environments in buildings. It is no longer tenable for architects, engineers, physicians, microbiologists and epidemiologists to ignore the impact of bioburden spread by ventilation on productivity or incidence of disease due to infection or allergy in hospitals.