The Russian space station Mir had problems with fungi growing on and damaging the station's equipment. Once the crew is up on the Space Station, all their air and water and the surfaces that they touch must be kept clean. The air in the Station is kept moving, and it all passes through filters to clean it. Dry air also slows down the growth of microbes, so the Station's air does not have much water in it. Water is heated to kill any germs, and then it is treated with iodine to make it much cleaner than what we drink at home.
Fungi come in all kinds of sizes and forms.
Yeast, used for baking bread is a tiny, one-celled fungus. Mushrooms are fungi we buy at the supermarket and enjoy eating. Microbes must also be kept from growing on surfaces of the Station. This is for the health of the crew, and also to guard the Station's hardware. Microbes are a real danger to the Station's material because they will eat almost anything.