Molds are fungi. Molds in water are mostly microscopic and single-celled, although they can grow into sheets of slime and larger structures like mushrooms if given the chance. They often contaminate water-using equipment via airborne spores. They are a major source of the “musty-earthy-mildewy-moldyfishy” T&O compounds that are also produced by algae and filamentous bacteria. With one exception, molds are harmless, and the only problems” they cause are moldy T&O and ugly slimes. The exception refers to people with damaged immune systems, who can die from some fungal infections.