British soldiers fighting in the
Indies had a serious problem with Malaria. The British also had a
tonic water that contained quinine, which was effective at fighting
malaria. The tonic water tasted terrible, so the British soldiers
mixed gin with it to make it palatable. Upon returning home after
the war, they continued to drink the mix and it became quite
popular.
Incidentally, Malaria comes from the Italian, meaning 'bad air' as
it was originally thought to be caused by dirty air.
Tonic water still contains some quinine, but much less than the
original, and now usually has artificial sweeteners to moderate the
bitter taste. Interesting to note the sensitivity of quinine to UV
makes it appear fluorescent in direct sunlight and glows blue under
black lights.