Fish live in water. This means that in practice they are swimming in everything that could potentially cause diseases.
The skin and mucous membranes are the only things that separate the inside of the fish from their surroundings, and an intact barrier of skin will therefore be key to the first-line defences of the fish against disease.
This barrier of skin can be weakened by a variety of different means, of both mechanical and infectious origin. In addition to the significance of a rupture in the skin barrier in terms of resistance to disease and fluid balance, wounds in the skin of the fish will be a challenge in terms of fish welfare.