Smallpox was special. It produced visible symptoms in each infected person, so cases could be identified and isolated while health workers vaccinated everyone around them, stopping viral spread by containment; and it dwelt within no other reservoir than humans, so it couldn’t reemerge from a nonhuman animal host. Polio eradication has been harder. That virus too inhabits no other host, but many polio infections are asymptomatic, making them harder to identify and isolate. Eradication of SARS-CoV-2 will almost certainly be impossible, again because it transmits from asymptomatic carriers, among
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