"The Ki, as you know" - Frock always assumed his listener was as familiar with a subject as he was - "at one time used the bark of a certain bush as a headache remedy. Charriere studied them i 1869 and noted their use of this bush in his field journals. When I showed up three quarters of a century later, they had stopped using the remedy. They believed instead that headaches were caused by sorcery." He shifted in his wheelchair.
"The accepted remedy was now for the kinfolk of the headache victim to identify the sorcerer and, naturally, go off and murder him. Of course, the kin of the dead sorcerer were then required to avenge this death, so they often went right back and killed the person with the headache. You can imagine what eventually happened."
"What?" <...>
"Why," Frock said, spreading his hands, "it was a medical miracle. People stopped getting headaches."