The immune system is the body system that protects humanoids from disease and illness.
To allow for a healthier life, Jaffa are given a young Goa'uld symbiote to supplement the immune system in healing the host whenever harmed or to protect them from a disease. However, if they remove the symbiote and do not return it within a sufficient amount of time, then the Jaffa will eventually die. (SG1: "Children of the Gods", "Thor's Hammer")