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And variola’s genome is so large that it is exceedingly difficult for even experts to assemble.

How the loss of genes would have made variola more virulent is not exactly clear, however.

Monkeypox is a poxvirus in the same family as variola – the virus that causes smallpox – and cowpox viruses and likely evolved in animals before jumping to humans.

The variola virus, which causes smallpox, is the only disease to have been eradicated by human medicine.

The C.D.C.’s campus in Atlanta is home to one of two Level 4 labs left in the world that harbors the live variola virus, which causes smallpox and was declared eradicated globally in 1980.