Why was smallpox a good candidate for eradication?

Describe at least three ways that an infectious disease, like smallpox, can significantly impact history.   Why was variola virus such a dangerous pathogen?   How did developments in the prevention of smallpox help lead to our current practice of immunization? Why was smallpox a good candidate for eradication?  What were the challenges to eradication?

250-300 words

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