Fungal diseases are often caused by fungi that are common in the environment. Fungi live outdoors in soil and on plants and trees as well as on many indoor surfaces and on human skin.Most fungi are not dangerous, but some types can be harmful to health.spergillosis-Caused by the fungus Aspergillus and usually occurs in people with lung diseases or weakened immune systems. 1  Symptoms. 2  Risk and Prevention. 3  Diagnosis and Testing.