To simplify considerably, helper-T cells occur in two main types, one that targets microorganisms and one that targets parasites. The body cannot sustain large numbers of both, and hence adult immune systems tend to be skewed toward one or the other, usually depending on whether as children they were more often exposed to microorganisms or parasites. Indians have historically been burdened by flukes, tapeworms, and nematodes, so they have long had majorities of parasite-fighting helper-T cells. Europeans, who grew up in germ-filled environments, usually lean the other way. As a result, the
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