In the days when there were more white farmers in Africa, I heard tell of a farmer’s wife who scolded her house boy (as they were then called) for swatting a fly with the knife he was using to cut up meat.

Usually obedient, he was clearly puzzled by being taken to task on this occasion. Patiently she explained that flies were covered with bacteria: little creatures that could get inside you and make you ill.

“But Mistress,” he protested. “Surely we Christians don’t believe such a thing?”