What this video covers
The clip explains why growth inside an HVAC system is the worst location for it.
Mold anywhere else stays roughly where it is. Mold in the ductwork is actively distributed, reaching every room served by a supply vent each time the system runs.
The takeaway
Duct contamination converts a local problem into a whole-house exposure. If symptoms are present in every room rather than one, the air handler is the first thing to look at.
Read the full written guide: Mold in HVAC Ducts.