Three men armed with machetes killed an 8-year-old albino boy in Burundi last week and are believed to have smuggled his limbs to Tanzania, where witch doctors use albino body parts for potions. At least 35 albinos were killed in Tanzania in 2008, prompting police officials to set up an emergency hot line and a program to distribute free cell phones to all albinos. How many albinos are there in Tanzania?

A whole lot. Albinism, a genetic disorder characterized by lack of melanin pigment in the skin, hair, and eyes, is listed as a rare disease by the National Institutes of Health—meaning it affects fewer than 200,000 Americans. Around the world, between one in 17,000 and one in 20,000 people are albinos. The prevalence in parts of Africa, however, is far higher than the global average. Albinos make up about one in 4,000 people in South Africa and perhaps one in 5,000 in Nigeria. According to a 2006 review published in the journal BMC Public Health, the prevalence in Tanzania is one in 1,400, but this estimate is based on incomplete data. Since Tanzania’s total population is more than 40 million, that would suggest an albino community of about 30,000. A census is under way, however, and the Albino Association of Tanzania believes the total figure could be more than 150,000.

Albinism may be more prevalent in some geographic areas because of inbreeding. A study published in 1982 notes that albinism is less common among the South African Zulu and Xhosa tribes (one in 4,500) than the Swazi and Sotho-Tswana tribes (one in 2,000), which have no taboo against cousins marrying. In Zimbabwe, about four-fifths of albinos belong to the majority ethnic group, the Shona. Since the Shona discourage consanguineous relationships, this prevalence may be the result of the founder effect, wherein a small number of people from a larger population form a new community, resulting in the loss of genetic variation. But the extraordinary rate of albinism in Tanzania is not yet fully understood.

In any case, albinism is especially dangerous in sunny climates, like Tanzania’s, because lack of melanin predisposes albinos to severe skin damage from UV exposure. Albinos frequently suffer from sunburns, blisters, and solar keratosis as well as visual problems like myopia.

Bonus Explainer: How do you count albinos? Look to the children. Researchers distribute surveys at schools and conduct interviews with administrators to get a sense of the albino population among the pupils, then extrapolate to the rest of the population. Hospital maternity wards are sometimes targeted, too, to count the number of albino babies born. And Albino Associations conduct outreach programs to register the local population officially.

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Explainer thanks William Oetting of the University of Minnesota.