What this video covers
The clip traces how a problem in one bathroom reached every room in the house.
Spores were drawn into the return vent, passed through the air handler, and were then delivered to every room with a supply vent. The heating and cooling system turned a contained problem into a whole-building one.
The takeaway
If your HVAC has been running while mold was active anywhere in the house, the ductwork is part of the investigation. Remediating the source room alone leaves the distribution system loaded.
Read the full written guide: Mold in HVAC Ducts.