From Kwame Anthony Appiah's Cosmopolitanism: Ethics in a World of Strangers:

There’s an oft-told anecdote about a medical missionary in a remote place, who watches, in horror, as people give untreated well water to their babies. The children regularly get diarrhea, and many of them die. The missionary explains that, even though the water looks clear, there are tiny, invisible creatures in it that make the children sick. Fortunately, she says, if they boil the water, it will kill these bacteria. A month later she’s back, and they’re still giving the babies the dirty water. After all, if a stranger came into your community and told you that your children got influenza because of witchcraft, would you respond by going out and slaughtering a sheep? Then the missionary has another idea. Look, she says, let me show you something. She takes some water and boils it. See, she says, there are spirits in the water, and when you put it on the fire they flee: those bubbles you see are the spirits escaping, the spirits that are making your children sick. Now boiling water makes sense. Now the babies stop dying. In belief, as in everything else, each of us must start from where we are.

When people get sick for unaccountable reasons in Manhattan, there is much talk of viruses and bacteria. Since doctors do not claim to be able to do much about most viruses, they do not put much effort into identifying them. Nor will the course of a viral infection be much changed by a visit to the doctor. In short, most appeals in everyday life to viruses are like most everyday appeals to witchcraft. They are supported only by a general conviction that sickness can be explained, and the conviction that viruses can make you sick.

If you ask most people in Manhattan why they believe in viruses, they will say two kinds of things: First, they will appeal to authority. “Science has shown,” they will say, though if you ask them how science showed it, you will pretty quickly reach an impasse (even with scientists, by the way, unless they happen to be virologists unusually curious about the history of medicine). Second, they will point to phenomena—the spread of HIV or the common cold, the death of their great-aunt last winter, a picture of a virus they once saw in a magazine—where the viral theory explains what happened.

Similarly, in Kumasi, people who are asked why they believe in witchcraft will appeal to authority, too. “Our ancestors taught us about it.” And they will then go on to tell you of cases of witchcraft they have seen or heard of, filling in for you all the things that it explains. Sir Edward Evans-Pritchard, one of the greatest anthropologists of the twentieth century, wrote a wonderful book called Witchcraft, Oracles and Magic among the Azande, about a people of that name who live in the Sudan. Having explained their ideas about witchcraft in great detail, he observes at one point that sometimes, in the evenings, when he saw a flash of flame in the bush around the Azande settlement where he was living, he found himself thinking, “Look, a witch.” Of course, he didn’t believe it, really. He knew it was probably someone from the village going off to relieve himself, carrying a flaming torch to guide him on his way. But what he was teaching us is that what you see depends on what you believe. What it’s reasonable for you to think, faced with a particular experience, depends on what ideas you already have.