Fungal spores are in the air all around us.
They need protein to feed them, this comes from mites, parts of mites, skin, anything.
They can attack coatings and glass.
Very old glass types are less affected, so 100 year old lenses may survive bad conditions.
Special glass types like ED, fluoride containing glasses, and other exotic types are very easily affected and maintain their integrity with very special coatings.
In a triplet APO, the difficult exotic glass is usually in the centre of the objective and well protected in its high quality cell.
Fluorite doublets have the special element behind a tough front element.
Early fluorite crystals, did just that, crystallised.
So one has to be careful buying secondhand exotics.
If these have been disassembled by an amateur, there could be problems, and also they need to be nitrogen purged again, which amateurs may not be able to do.
But modern high quality equipment is usually properly made, but I have seen modern cheap waterproof optics full of humidity when new.
So it is worth carefully checking any new or secondhand optic.
On a low humidity day it might seem clear, but display internal misting in high humidity.
Also when getting something new by mail order it may initially be very damp.
