woman developed a severe eye infection thanks to a rare virus — cowpox, a cousin of smallpox, which she contracted from her pet cat.

Samples from the cat's lesions and the woman's eye both tested positive for orthopoxvirus — the family of viruses that includes the smallpox virus (variola virus), cowpox virus and monkeypox virus.

It's closely related to the vaccinia virus, which is used in the smallpox vaccine.

(The physician Edward Jennor famously used cowpox to create the world's first vaccine, against smallpox, in 1796, after he noticed that dairy farmers who contracted cowpox were immune to smallpox.) Today, cowpox is rare in cattle, and the main reservoir is rodents, according to the Merck Veterinary Manual.

Cats can become infected when they kill rodents carrying cowpox, but transmission from cats to people is rare, Kiernan told Live Science.

Humans can become infected with cowpox through contact with cowpox lesions on cats' skin, but the virus isn't very contagious between people and cats, and the risk of infection can be greatly reduced with hygiene measures such as wearing gloves when handling infected cases, according to VCA Animal Hospitals.

Doctor's suspect the woman's eye became infected when she petted her cat and then touched or rubbed her eyes, Kiernan said.

Kiernan added that he and his colleagues have never seen a case of cowpox infection of the eye before, and few cases have ever been reported in the medical literature.

(Smallpox was eradicated from the world in 1980, thanks to a global vaccination campaign, but governments worry that the smallpox virus could be used as a bioweapon, Live Science previously reported.).

Although smallpox has been eradicated, "orthopoxviruses remained in certain parts of the world, including cowpox in Europe, and monkeypox in central and west Africa," Kiernan said.

"I suspect human cowpox cases will remain a rarity, however I do wonder whether we could potentially see more cases in the future now that populations are immunologically naive," said Kiernan, referring to the fact that people are no longer routinely vaccinated with the smallpox vaccine, which may have provided some protection against other orthopoxviruses.