"There are two basic types of disease immunity: passive and active. Passive immunity is resistance that is given to - not developed by - the creature carrying the defense. Such "outside help" provides resistance to a disease in a very short time, but that protection will eventually disappear. In some ways, passive immunity is like a youngster's winter snow fort. The easily erected play castle can provide protection during a short "battle" ... but it will melt away after a few days of sunshine. "