There are lots of people who are in favour of using salt, and other people who don't use it at all. I have never used salt and I don't really think it's necessary in freshwater aquariums.
If you follow some basic rules, you can help to prevent disease occuring in your aquarium. Carry out regular water changes, be very careful when adding new fish, get your fish from a reputable store, preferably one that you have been using a regular basis. Feed your fish and good quality food, try and avoid rapid temperature changes if Possible.
Obviously with all the will in the world, fish to become ill.