Penicillin can wipe out syphilis with just one shot. As the antibiotic came into wide use in the 1950s, the number of syphilis cases and syphilis deaths plummeted. And that’s when teen pregnancies and illegitimate births began to rise — long before the invention of the birth control pill.

That’s the provocative thesis of Andrew Francis, an economist at Emory University in Atlanta who studies HIV/AIDS and the cost of disease. He knew that the rate of new HIV infections spiked after antiretroviral treatments used widely after 2000 made the disease less of a death sentence.

Francis was browsing through historical trends in gonorrhea rates one day, when he saw that the rate of this non-deadly STD really started cranking up in the early 1950s. “Really?” Francis said to himself. “What was going on in the 1950s?”