I'm aware you were talking about malaria.
The amount of quinine British soldiers in India used to take was dozens of times larger than in any modern tonic drink.
The tiny doses in tonic water do absolutely nothing to malaria. It's simply a historical leftover, saying this as a big fan of tonic myself.
"...for those interested, is according to Meshnick’s chapter in “Malaria: Parasite Biology, Pathogenesis and Protection (1998, ed Sherman) that modern tonic water contains 15mg/L quinine, and so has little antimalarial effect. However, Meshnick also mentions that drinking a glass of a decoction of Cinchona bark (made by soaking the bark in brandy or gin for 5 or 6 days) two or three times a day would have provided at least 1-2g quinine per day. Meshnick says that this would have protected against malaria.
Therefore, consuming 67 litres of tonic water would provide 1g quinine. That means a lot of G&T’s, and there are definitely easier ways to prevent malaria."
