A simple cold germ could multiply until it killed you if your immune system didnt stop it. Germs enter your body several times per day. Any one of them could kill you. Your body is constantly under attack from free radicals that mutate cells. Macrophages seek out these mutating cells and kill them.
Since the immune system is the priority system, it gets your body resources first when you are under attack. Think about how you feel when you are ill. The majority of how you feel isnt from the germ in your body; it is from the reaction of your immune system. Your immune system uses vitamins, minerals, cellular energy, oxygen, hormones, and many of the other body resources. When your body is under attack, the immune system drains the rest of your body of these resources causing you to feel tired and weak. Even a healthy person needs outside assistance to help his immune system, which is daily working hard to keep the individual healthy.