When moisture-laden air at dew point temperature penetrates inside a cavity of the structure, it condenses into liquid water on that surface.
HVAC ducts within interstitial spaces (chases) can leak out cold air through unsealed joints/connections which produces dew point surfaces.
In addition, the cool ducts themselves can condense humid air and “sweat” even more liquid water into the interstitial spaces thereby exacerbating mold and bacteria growth.
It is physically impossible to build envelope assemblies so that they completely prevent air infiltration, exfiltration of water vapor diffusion.
Building professionals have moisture sensing instruments to discover areas of interstitial condensation which may contain possible mold & bacteria growth.