This subject is important as it could be called financial loss prevention or pain and suffering prevention.
Prevention of disease boils down to two categories. Primary prevention includes vaccination, good nutrition and avoiding other horses with disease (quarantined). Secondary prevention involves removing causes such as stress from overcrowding, removing fly breeding sources, and cleaning up filthy environments.
Disease prevention requires planning and playing by rules. When laziness and shortcuts occur, then diseases are more likely. All diseased horses are associated with pain and suffering for the horse and the owner. It also costs money to fix, or in the worst case, it causes the loss of the investment (purchase, maintenance and training). So don’t be lazy = save money plus enjoy the horse and your sport.