What I think most the posts right now are missing that condensation itself isn’t enough to sustain life. Moisture is only one part. There is a lot of dust in the air that gets sucked into the vents. Usually hvac systems blend a mix of inside and outside air. Indoor air has a lot of things like skin cells, fibers from clothing and carpets. Outdoor air has dust air pollution pollen and other stuff.

Commercial hvac systems in theory have filters, but they aren’t perfect filters, and roof mounted systems are notorious for having old dirty filters, missing or not properly placed so they intake a lot of unfiltered air.

All that stuff settles in the vents, add moisture and it’s a perfect combination for bacteria to grow. Especially in poorly maintained systems they might have other mechanical issues, such as condenser coils that don’t run a defrost cycle long enough or often enough to prevent the formation of ice or excess condensation build up. Burnt out uv lamps in some commercial hvac systems.