ARI SHAPIRO, HOST:
Sometimes great successes have unintended consequences. That's the case with one of the world's biggest triumphs - the eradication of smallpox. That achievement may have set the stage for an outbreak of another kind - one we're dealing with today. And now there's new evidence to back that theory up. NPR's Gabrielle Emanuel reports.
GABRIELLE EMANUEL, BYLINE: Eugene Bangwen spends his days thinking about mpox formerly called monkeypox.
EUGENE BANGWEN: There's been an exponential rise in the number of cases of mpox.
EMANUEL: He studies infectious diseases at the Institute of Tropical Medicine in Belgium. In the past few years, there have been over 100,000 confirmed mpox cases globally. That's many times higher than previous years. Bangwen has questions.
BANGWEN: I mean, where did it come from?
EMANUEL: Why are the numbers climbing so quickly? Why is mpox showing up in new places?
BANGWEN: These are all questions that we do not understand.
EMANUEL: Researchers think there are a lot of factors at play. Bangwen decided to look back at all the confirmed mpox cases from the Democratic Republic of the Congo in recent decades to see what he could find.
BANGWEN: We charted everyone - the ages of everyone.
EMANUEL: And in this data, he could see that almost all of the mpox patients were born after a certain point in time. Bangwen says to understand why that's the case, you have to go back to 1970 to a remote corner of the Congo, deep in the rainforest. It was there that a mother took her 9-month-old baby boy on an ill-fated trip to visit Grandma.
WILLIAM MOSS: So it was shortly after the child arrived in that village of his grandmother's where he became ill.
EMANUEL: William Moss is an epidemiologist at Johns Hopkins at Bloomberg School of Public Health.
MOSS: He became sick with fever and two days later developed a rash.
EMANUEL: Moss says that rash set off alarm bells because it looked an awful lot like smallpox. The world was getting very close to eradicating smallpox, and in order to prove it was really truly gone, every single patient with a suspicious rash or lesion needed to be investigated. So at a local hospital, a doctor took scabs from the youngster and sent them to a lab in Moscow. The answer came back - this wasn't smallpox. This was monkeypox.
MOSS: That was a surprise, obviously, to everyone.
EMANUEL: It was the first known human case. That little boy, he was patient zero. Soon scientific investigators showed up.
MOSS: They interviewed the family members. They interviewed neighbors. They tried to identify, you know, contacts and routes of transmission.
EMANUEL: One thing they discovered, this little boy was the only one in his family who hadn't been able to get the smallpox vaccination. This was the first clue that smallpox and mpox are so closely related that the smallpox vaccine gives people some immunity against mpox.
MOSS: As more and more cases of monkeypox were identified, it became clearer that these cases were occurring in people who had not been vaccinated against smallpox.
EMANUEL: But at that time in the 1970s, almost everyone was vaccinated. So the total number of mpox cases was very low. But then came 1980. Smallpox was declared eradicated and soon vaccinations ended. When Eugene Bangwen looked at all the data in his study, he could see the impact of stopping those vaccinations. It was like an invisible line in the sand. People born before that time got immunity to mpox. People born after that time didn't get immunity. The end of smallpox vaccinations was the beginning of mpox, and it set the stage for the surge in mpox cases we see today. Bangwen points out another lesson from this history - the power of vaccination.
BANGWEN: There is a strong need, therefore, to vaccinate as a means to control mpox. Yes.
EMANUEL: Vaccines were key to overcoming smallpox. He says the same is likely true of mpox. Gabrielle Emanuel, NPR News.
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