Immunity-The immune system, which is designed to protect a human from invading pathogens, is extremely complex. When everything is functioning in synchrony, the system works well. The problem is that many things can compromise the immune system, and when that happens, the horse is at an increased risk of developing disease. Often one (or more) of three key elements are at the root of the problem when the immune system becomes compromised, says Glen Gamble, DVM, of Riverton, Wyo. They are stress, nutrition, and age.