Final answer:
Alicia remained healthy because she had lifetime immunity from a previous chickenpox infection. The immune system's memory and the chickenpox vaccine contribute to preventing reinfection and spread within the population.
Step-by-step explanation:
Alicia remained healthy despite close contact with her son John during his chickenpox illness because she had already contracted the disease as a child. This previous infection provided her with lifetime immunity to the varicella-zoster virus, which causes chickenpox. Once a person recovers from chickenpox, their immune system typically remembers the virus and protects them from getting sick again if they are exposed to it. Furthermore, the availability of a chickenpox vaccine has made it much easier to prevent the disease and its spread within populations, thereby reducing the incidence of both chickenpox and shingles, which is caused by the reactivation of the dormant varicella-zoster virus.