Airborne Transmission of Communicable Infection — The Elusive Pathway

What does it mean to describe an infection as airborne and what are the clinical implications? The concept of the air we breathe as a reservoir for disease-causing agents harkens back to the pre-Pasteur teachings of "miasmic-induced disease" of Sir Edwin Chadwick. Modern germ theory largely rejects the vague concepts of airborne vapors as a cause of disease, and has focused on obvious transmission patterns of infectious agents from the source to a susceptible host. Observable mechanisms of transmission such as contaminated water, direct surface contact, and large droplet sprays, can all be validated as means of disease communicability. In addition, many of the aerosol-transmitted disease agents such as smallpox and measles have been effectively eliminated or controlled through aggressive preventive and primary care of the last century. As a result, the aerobiological basis of disease transmission has faded somewhat into the more obscure theoretical milieu. Only tuberculosis frequently defied directly observable transmission by close contact. Thanks largely to the work of William Wells and Richard Riley, we now know that tuberculosis is naturally transmitted only by the airborne route. But proof of airborne transmission was an arduous task, requiring continual exposure of a guinea pig colony to active tuberculosis cases in a clinical ward. Only after satisfying Koch's postulate by demonstrating disease agent transmission from the infected patient to a naive animal, was there recognition that tuberculosis was, in fact, an airborne-acquired disease. This article reviews the possible airborne transmission of acute respiratory system (SARS) at the Amoy Gardens apartment complex in Hong Kong. The SARS epidemic provides an opportunity for the critical reevaluation of the aerosol transmission of communicable respiratory diseases.