What this video covers
Condensation on a cold window is not just water on glass. Warm indoor air meets a cold pane, drops below its dew point, and leaves moisture sitting on the frame and sill every day through winter. That is a daily water source resting on painted wood and drywall.
By spring the material has been damp long enough for mold to establish, usually along the bottom of the frame and the wall directly beneath it. The clip explains why the problem stays invisible until warmer weather speeds growth up.
The takeaway
Wiping the glass treats the symptom. Condensation forms because indoor humidity is high and the glass is cold, so the fix is lowering humidity and raising the surface temperature of the pane. Stop the condensation and there is nothing left to feed spring growth.
Read the full written guide: Winter Window Condensation and Spring Mold.