What this video covers
This is the top end, and the clip walks through why a basement gets there.
The whole floor is involved, structural timber has been compromised rather than just stained, and the HVAC system has carried spores into the rest of the house. At that point the job is not remediation of a room, it is remediation of a building.
The takeaway
Basements reach this scale because they combine constant moisture with materials that hold it and an air handler that distributes the result. It is the single most expensive place in a house to ignore a damp problem.
Read the full written guide: Basement Mold Remediation Guide.