lens fungus

[Raven Elliott]

I was wondering how a lens gets fungus. There is a live butterfly exibit a local museum, and the room is very hot and humid. Will this contribute?

[Arne Croell]

Fungus spores are ubiquitous. Even if the lens is assembled in a cleanroom that filters them out, they will get inside later since lenses are not hermetically sealed. If the conditions are right (rel. humidity over 70%) they will eventually germinate and grow. But that is for long term exposure. A visit of several hours won't do any harm, I assume.