Camera "bags" are usually not airtight (they're "breathable"), so will allow moisture to evaporate/escape over time. Depending on what a bag is made out of, and how it's designed, some bags are more breathable than others.
Water-proof cases (such as Pelican) and other hard-shell cases (such as shipping cases) tend to hold moisture in. When closed, this trapped moisture can promote mold to grow in lenses, causing permanent damage. When possible, keep the case's lid cracked open (and the cases' pressure release valve, too) so moisture can escape.
When waterproof/hardshell cases are closed, put dry silica-gel packets or canisters inside the case to absorb moisture. Silica-gel packets are available in a wide variety of sizes from camera dealers. Periodically the silica-gel must be replaced or refreshed (baked/dried), or it won't be able to absorb moisture.
A tip: I keep a #1520 Pelican case containing my camera & lenses cracked-open slightly when not in use (with the latches not latched) for ventilation. However, to prevent the inevitable accident of lifting the case by the handle and spilling the contents on the floor, I thread a lanyard through the padlock holes on the case, and loosely tie the lanyard in a knot, as a safety. That way the contents of the case can "breath", I won't break my lenses, and when home my cats won't mess with the contents.
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