In this lecture, students learned how microorganisms like bacteria and fungi act as decomposers by breaking down dead plants, animals, and waste into nutrients that return to the soil, helping plants grow and keeping the environment clean. Decomposition works faster in warm, moist, and oxygen-rich conditions, and while it has benefits like recycling waste, making compost, and enriching soil, it can also cause bad smells, spread diseases, and spoil food. The lecture also covered communicable diseases—illnesses that spread through air, water, food, direct contact, or animals—along with examples such as flu, cholera, typhoid, malaria, and ringworm. Students learned ways to prevent infections through personal hygiene, clean surroundings, safe food and water, vaccinations, and avoiding contact with sick people or animals.