Most people are aware that mosquitoes are the deadliest animals in terms of total human deaths, second only to humans themselves. This is due to the malaria parasite they spread. It's estimated that four to five percent of all humans who have ever lived died from malaria, not half as some sources claim. The treatment for malaria is quinine, known since the 1700s and often found in tonic water, which is bitter and not very palatable. The anecdotal story goes that during British colonization of India, the British East India Company faced malaria issues. Since drinking tonic water was unpopular, they added gin, creating the gin and tonic. Modern research shows that a significant amount of tonic water (~1 liter) is needed for a minimal effect, making the story largely apocryphal. However, it's interesting to think that we turned the treatment for one of humanity's worst diseases into a cocktail.
We no longer use this treatment because it had a mortality rate of 15%, although this was much lower than the death rate for syphilis. Unfortunately, many parts of the world still suffer from malaria, where it remains a major killer.