When you get an acute infection, such as influenza, the body generally responds with a coordinated response of immune-cell proliferation and attack that rapidly clears the pathogen. Then, their mission done, the immune system stands down, leaving a population of sentinel memory cells to rapidly redeploy the immune system in the event of reinfection. This is why vaccination works, and it's why, in theory at least, people who have had the chicken pox once will never get it again...
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