How do you get rid of mold, and when do you need a professional?

The essential principle is that mold is a symptom. It cannot grow without moisture, so cleaning without finding and fixing the water source, a leak, condensation, poor ventilation, or humidity above roughly 50 percent, guarantees it returns. The EPA's practical threshold is about 10 square feet: smaller areas on hard surfaces are a reasonable do-it-yourself job using detergent and water, scrubbing thoroughly and drying completely, with an N95 respirator, gloves, and eye protection. Larger areas, contamination in HVAC systems, or growth following sewage or contaminated flooding warrant professional remediation.

Two common mistakes are worth correcting. Bleach is widely assumed to be the answer and is a poor choice on porous materials like drywall and wood, because its water content soaks in and feeds regrowth while the chlorine stays on the surface; detergent and physical scrubbing work better, and porous materials that are heavily colonized usually need to be removed rather than cleaned. And painting over mold accomplishes nothing except hiding it, since it continues growing beneath. On the health question, keep expectations calibrated: mold exposure commonly causes allergic and respiratory symptoms and is a genuine problem for asthmatics and immunocompromised people, while the dramatic claims made about toxic black mold specifically have far weaker evidence than their prominence suggests. Any visible mold should be removed regardless of species, which is also why paid species testing rarely changes what you do.