It has to do with Jesuit bark, found on trees in the tropical Andes, which was discovered to be one of the first breakthrough treatments for malaria in the 17th century. In those days, malaria was everywhere and it was bad. Really bad. (Sidenote: There was malaria in New Orleans as recently as 1883, and the disease wasn't fully eradicated in the U.S. until the 1950s.) So, anyway, Jesuit bark was a big deal.
“Just a huge, huge advance in medicine,” said David Morens, a senior scientific advisor at the National Institute of Allergy and Infectious Diseases. “But nobody knew in those days that malaria was a specific disease. It was just a fever. Now, with Jesuit bark, this was a fever that could be cured."
Turns out the active ingredient in this tree bark was quinine—you may know it as tonic—which doctors promptly became obsessed with. They started prescribing it for every fever, malaria or not. And that’s why people still drink the stuff to settle their stomachs today. “Gin and tonic! It’s a holdover from that era,” Morens told me. “The Brits used to drink gin and tonic to have fun getting high and to cure malaria while doing it.”
Update from a reader:
My love of both gin & tonics, and hyperlinking to related content for further reading compels me to write in. The other day, Adrienne LaFrance posted a Note about how the discovery of quinine’s malaria fighting properties lead to the gin and tonic. There seemed to be a small flurry of articles and blogpost about the connection between malaria and the gin and tonic a year or two ago (for example, from NPR’s The Salt), with some going further to discuss quinine’s role in the British Empire’s expansion (see Slate and The Dish, with The Dish quoting The New Republic). More linking on Notes to related pieces on the web for those that want to explore the topic further please.