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Thanks. Twitter will use this to make your timeline better. UndoUndo
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I think your second tweet pretty neatly resolves the question raised by the first - We know that exposure to related viruses (Variola Minor, Cowpox) confers protection against Variola Major.
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So, since V.Maj. is the "youngest" of those mentioned, it might never have been exposed to a naive population prior to European contact with the New World.
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