Mold reproduces by releasing spores, thousands of tiny versions of itself that float through the air. Mold spores are normal and expected in small amounts, and don't necessarily mean there's an infestation. But if the spores can find consistently warm, wet areas in your home to land on, they can rapidly multiply and become a real problem. 

Breathing in too many spores causes cold-like symptoms in most people: nasal and sinus congestion, runny nose, difficulty breathing, chest tightness, cough, and sneezing. Sensitivity to mold varies, so some people may experience no symptoms while others experience potentially dangerous side effects, such as fungal infections. Most people will have more moderate sensitivity, but it's important to understand that as long as mold is present, it will continue to spread, and initially mild symptoms can become more severe if the mold is not eliminated.