What this video covers
Mold spores are already in your house. They are in every house, drifting in on shoes, pets and open windows, and there is no realistic way to keep them out.
What the clip points out is that spores alone are harmless. They need one thing to stop being airborne dust and start being a colony, and that is moisture. Give them a damp surface and roughly 48 hours, and growth begins somewhere you cannot see it.
The takeaway
Do not think about keeping spores out. Think about denying them water. Every mold problem starts as a moisture problem, which is why leaks, condensation and humidity are the things worth watching rather than the air itself.
Read the full written guide: Signs of Mold in Your House.