Not all fungi cause disease. Yeast is a single-celled fungus that is economically important because we use it to make some bread and beer. Not all fungi are single-celled. Some such as mushrooms are multicellular and so much larger. These are still made of tiny cells like yeast, but have many more of them. Fungal cells have a nucleus and so are eukaryotic.
Athlete's foot is a rash caused by a fungus that is usually found between people's toes. It causes dry, red and flaky or white, wet and cracked skin. It is often found in communal areas like swimming pool changing rooms or gyms. It is transmitted by touching infected skin or surfaces that have been previous contaminated. It is treated by antifungal medication.
Rose black spot is caused by a fungus which infects roses. It infects leaves and causes black or purple spots on the leaves. The rest of the leaves often turn yellow and can drop off the plant. This reduces that plant's ability to photosynthesise and reduces growth. It can be transmitted in air or water, as well as through direct contact by gardeners. It is treatable using fungicides and by removing and destroying infected leaves.