Hi, Noted the history about an active person playing tennis, biking, running, has a room without ventilation and has mold below the sink, family members suspect that your immune system has gone down. Since you are playing so much but have not mentioned your symptoms as to how is it suspected to have reduced immune system nothing specific can be said about. Anyway, the molds, mildew and fungi in the house in particular can cause allergic reactions causing allergic bronchial asthma, allergic rhinitis and such problems and as such do not affect the immune system. Hence, please mention your symptoms when you put next time. Secondly, the thumb rule in medical field is prevention is better than cure You can certaily apply the solution that kills molds, do water-proffing in such a way that there is no seepage thus not allowing molds, mildew and fungi to grow and have healthy breathing. IF there are any symptoms then consult your Doctor and get treated accordingly. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. T. Chandrakant, ENT & General Surgeon