When I did the Winter Survival Course at Bad Kohlgrub in the 1970s, our FAK had two identical little bottles of pills, one containing Paludrine and the other water-sterilising tablets. Camped beside the raging River Ammer, downstream of both Unter- and Oberammmergau, we religiously added the water-purifying tablets to the water we took from the river. After three days, we were alarmed to discover that the water-purifying bottle was full, while the Paludrine bottle was empty. At least none of us (including a later CAS) got malaria!