Most fungi are saprophytic deriving nutrition from dead matter (organic compounds).  A few are parasitic; parasites requiring living hosts.

All fungi require an external source for organic material.  To obtain food, they extrude digestive enzymes and other substances which break down complex external nutrients.  Fungi then absorb the nutrients by "bathing" in the mixture of their secretions and moisture from the environment.  Moisture is vital for fungi precisely because they must bathe in their nutrients to absorb them.  Although fungi also need oxygen, removing moisture is the most effective method for halting growth.