two types of susceptibility. The first is the lack of acquired immunity—immunity gained from a previous exposure to a pathogen. People who have never had chicken pox are readily infected by the virus. After they come down with the disease, their immune system trains itself, so to speak, to fight off the virus, and they never catch it again, no matter how often they are exposed. Most Europeans of the day had been exposed to smallpox as children, and those who didn’t die were immune. Smallpox and other European diseases didn’t exist in the Americas, and so every Indian was susceptible to them in
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