There will, of course, always be some bacteria in even dry ventilation systems. These are primarily skin surface organisms recirculated from occupants, and outdoor bacteria borne on fragments of vegetation. On the other hand, if water is present in drip pans or even in ductwork, bacteria will grow and form biofilms in these places. In general, the colonies that are formed are populated primarily by Gram-negative bacteria that are, at most, opportunistic pathogens. However, they do contain endotoxins that can cause respiratory and ocular irritation, and can exacerbate asthma. The question remains: do these bacteria become airborne and enter the occupied space in sufficient concentration to cause these problems? This depends on the amount and thickness of biofilm, the location, and factors that would aerosolize the water along with surface bacteria. Actions within the system that might cause aerosolization include drips impacting on wet surfaces, sharp changes in airflow direction that lead to impaction on wet surfaces (which would also cause splashing), and any sharp movements of the ductwork. Of course, if there is sufficient water for bacterial growth, fungal growth is also inevitable. Fungal exposure combined with endotoxin exposure is more likely to lead to sensitization than the fungal exposure alone. Also, fungal spores are readily aerosolized from surfaces by all the factors mentioned above. We do not have data on the release of ventilation system bacteria into the occupied environment. However, the “possibilities” described here mandate repair of the cause of the water intrusion, and subsequent thorough cleaning of the system.