And that was possible in smallpox because of a few characteristics of the disease: one is that it's a--it's purely a human disease, humans are the only organisms that are affected by it.

天花能够被根治的原因在于,这种疾病具有以下几个特点,一是,它是一种只有人类才得的疾病,人类是唯一受天花感染的生物

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