She and Allan by H. Rider Haggard

Now it had been my intention to push forward across the river at once, but here luck, or our old friend, Fate, was against me. To begin with several of Umslopogaas' men fell sick with a kind of stomach trouble, arising no doubt from something they had eaten. This, however, was not their view, or that of Umslopogaas himself. It happened that one of these men, Goroko by name, who practised as a witch-doctor in his lighter moments, naturally suspected that a spell had been cast upon them, for such people see magic in everything.

Therefore he organised a "smelling-out" at which Umslopogaas, who was as superstitious as the rest, assisted. So did Hans, although he called himself a Christian, partly out of curiosity, for he was as curious as a magpie, and partly from fear lest some implication should be brought against him in his absence. I saw the business going on from a little distance, and, unseen myself, thought it well to keep an eye upon the proceedings in case anything untoward should occur. This I did with Miss Inez, who had never witnessed anything of the sort, as a companion.