Imagine your tribe lives in a place where your water is seasonally bad.

Once or few times a year a lot of people get sick and start shiting and puking themselves to death. So you bang some drums and waft some smoke over them to drive the bad spirits out.

Maybe half of them have a good enough immune system to weather it, so they recover. And just like the drum banging and smoke cure dysentery/cholera/typhoid, at least some times.