Answer:
John: he has naturally acquired active immunity to chicken pox from previous exposure.
Step-by-step explanation:
Your body naturally has some immunity to things that aren't you, but when you get sick with a specific pathogen, your body begins to make antibodies that specifically recognize that pathogen. Each antibody only recognizes one pathogen, and it floats around waiting to find it again so it can tell your body to kill it before you get sick again. This is why we become immune to chicken pox; after the first time you beat it, your body has the labeled antibodies and it can quickly destroy the pathogen before you get sick again.