Wikipedia defines fungus as “A fungus (/ˈfʌŋɡəs/; plural: fungi or funguses) is any member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds (British English: moulds), as well as the more familiar mushrooms.”
Funguses are very useful. They help create the bread and beer we eat and drink. They also do a great job of breaking down and decaying organisms, such as trees that have fallen etc.,
One of the downsides of living in a humid environment ( such as Ecuador) is that fungus grows quickly and on many surfaces.
When I lived in Tokyo I noticed that many camera stores sold “refrigerators” (basically humidity and temperature controlled boxes) for storing your camera equipment in.http://www.amazon.com/electronic-automatic-digital-control-Humidity/dp/B00870N90S I wish I had such a box, because I now have mold and fungus in my lenses.
It is expensive to have removed from the inside of the glass, and the glass will never be the same. I will likely be tossing my less expensive lenses because they would not be worth the expense of cleaning.