Molds are fungi that produce toxins to pollute their environment. Why do they do that? To defend their inhabitant and keep other molds from growing there - humans inhaling those are just collateral damage. In outdoor spaces where air can freely circulate, mold is no issue, even not in old-growth, moldy forests. Problematic are our modern houses, so well insinuated and built from impermeable plywood with no gaps where air can move through. Back in the days, log homes or houses build from purely wooden planks and stones provided enough gaps for air to circulate and help to avoid growth. That falls away today.
Mold then can poison you through their array of mycotoxins to the extent that molds can infest humans. Symptoms are often expresses based on their neurotoxicity in symptoms like depression, mood disorders and fatigue. All pretty common, yet unspecific issues.