Aquatic fungal infections are usually caused by the water mould Saprolegnia, and these infections are highly contagious and must be treated early if the fish is to recover. Usually fungal infections result in cotton-wool like growths on the fishes body, often on the mouth or fins. Fungal infections can also be caused by physical damage to the body.
The best course of treatment is to isolate the infected fish and use a proprietary anti-fungal medication such as API pimafix or eSHA labs 2000. Salt will have very little effect on the disease, and in fact the only thing that it will do is cause stress, which in turn will lower the fish's immune system even further.