Top: Cow-milker infected from the teats of a cow with natural cow-pox. Large depressed vesicle with a small central crust, tumid (swollen) margin, surrounded by well-marked areola and considerable surrounding induration (hardness associated with swelling)
Bottom: Same case, one week later. Reddish brown crust typical of recovering cow-pox cases, on a reddened elevated and indurated base.
Many mammalian species have members of the Orthopoxviridae that are specialized to exist within their systems - humans have smallpox, cows have cowpox, monkeys have monkeypox, and so on (note: chicken pox is NOT a member of this group - its name comes from an old English word meaning “Itchy”, and is completely unrelated).
However, sometimes, the similar viruses can cross species barriers, as in the case of cowpox. Though the viruses are specialized to their host species enough that they don’t easily spread between atypical hosts, they’re related enough that once an individual is infected with one pox virus, their immune system is able to to recognize and fend off the whole lot of them. This is why, with the assistance of cowpox (Vaccinia) cultures in administered vaccines (rather than all of humanity having to be in direct contact with cows…), smallpox was able to be eradicated in the wild.
A Text-Book of Bacteriology, including the Etiology and Prevention of Infectious Diseases. Edgar M. Crookshank, 1897.