Question

how does using killed or weakened bacteria in and immunization help hte body prevent infections?

Answer 1

see biology I answered it there...........

Answer 2

its more the principle applied for viruses ...
short version: in most cases your body already as the means to kill it by itself or to produce some sort of antibody against it, but it takes time!
giving a weaken form makes the body develop defenses that it "stores for a while" making more effective to fight against a "healty infection".