Yours is a fair assumption, but netting a fish can also cause it injury that can lead to fungus growth on the fish so you are damned if you do and damed if you don't.

As for releasing anything into your water, if the fish is in the bag with the malady and the water came from the same location as the fish and nothing is visible to the naked eye in the water, then you can just about bet if it is something that is detremental to the fish, the fish has already been infected.

That's the reason for quarentening new fish for at least a week before releashing the into an established community.