Tonic water used to contain quinine, a natural alkaloid which was for 300 years the antimalarial drug of choice. Gin was added to make the taste less bitter, and the drink was introduced in India during British colonial days. Quinine is still used in treatment of malaria but not as a first line drug. It also reduces fever and is a pain killer. Several hundred mg a day are needed to be effective.
One Gin and Tonic contains about 2 units of alcohol and used to contain 20 mg of quinine ( now it's just flavouring ). Even in the past, malaria would indeed be the least of your problems after bingeing on 50 G + T's
