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.
The fungus Erysiphe graminis causes a disease commonly called barley powdery mildew. It infects grass plants, including crops like barley and causes a fluffy white growth on the leaves. The disease is common in cooler, damper climates, such as in the UK. Eventually the plant stops being able to make chlorophyll and so cannot photosynthesise to make its own food. This leads to the death of the plant. The fungus produces spores in order to reproduce. The spores can be spread by the wind. Powdery mildew is treatable using fungicides and by removing and carefully destroying infected leaves.