In some cultures, blood loss is perceived as a danger not only to the individual but also to the larger community. George journeys to a remote Hindu village in western Nepal, where she finds Radha, a sixteen-year-old chau, which means “untouchable menstruating woman” in the local dialect. During her period, Radha can’t enter her family’s house or her temple, and she can’t touch other women, lest they be polluted. If she so much as consumes buffalo milk or butter, the buffalo themselves will get sick and stop producing milk. She can be fed only boiled rice, thrown by her little sister onto a plate from a safe distance, “the way you would feed a dog.”

Customs that denigrate women during menses are widespread. George notes that our word “taboo” is believed to derive from one of two Polynesian words: tapua, which means “menstruation,” or tabu, which means “apart.” Not long ago, in America, it was thought that “the curse” could cause women to spoil meat if they came in contact with it. But menstrual blood is not always seen as harmful, and menstrual segregation at its most benevolent can take the form of communality. Some three hundred miles northwest of where Radha lives, near the border between Pakistan and Afghanistan, menstruating Kalasha women “retire to a prestigious structure called the bashali, where women hang out, have fun, and sleep entwined,” George writes. “In this reading of menstrual seclusion, the woman is prized for her blood, because it means fertility and power.”

In Wogeo, an island off the north coast of Papua New Guinea, menstrual blood is held to be both lethal and cleansing, and men emulate menstruation by cutting their penises with crab claws. In ancient Rome, too, menstrual blood was not just a curse. Pliny the Elder wrote in his “Natural History” that when women had their periods they could stop seeds from germinating, cause plants to wither, and make fruit fall from trees. But their destructive power had its uses. A menstruating woman was able to kill a swarm of bees or ward off hail and lightning. Wives of farmers, Pliny suggested, could even offer a sort of pesticide: “If a woman strips herself naked while she is menstruating, and walks around a field of wheat, the caterpillars, worms, beetles and other vermin will fall from off the ears of corn.”

In Islam, menstruating women are forbidden to recite certain prayers and must refrain from vaginal intercourse. In Judaism, too, menstruation can be a cause of ritual impurity, as can childbirth. According to the rabbi and theologian Shai Held, “Childbirth takes place at—and to some degree unsettles—the boundaries between life and death: A new life comes into the world, but blood, considered the seat of life, is lost in the process.”

Observant Jews and Muslims alike follow dietary laws that forbid the consumption of blood. Both kosher meat and halal meat must be drained of blood, and kosher meat is also salted, to remove any residue of the substance. A tiny blood spot in an egg renders it inedible. While believers accept these prohibitions as divine edicts to prove devotion, some scholars speculate that they developed as health measures to prevent spoilage of meat, which is accelerated through oxidation and bacterial growth. These days, even meat that is not kosher or halal is drained of blood. People who say they like their steak “bloody” are actually responding to myoglobin, a red-pigmented protein that stores oxygen in muscle and brightens when exposed to air.

Yet, despite the firm proscription against ingesting blood, one breakaway Jewish sect of the first century A.D. made the idea of doing so central to its rituals. Its leader, Jesus of Nazareth, told his disciples that the bread and the wine at the Last Supper were his body and blood, and should be consumed thereafter in memory of him. The ritual of the Eucharist became a cornerstone of early Christianity, and with it the doctrine of transubstantiation—that a literal, not just figurative, transformation occurred during the sacrament.

“So then I’m, like, ‘Waaaaah! Waaaah!’ ”

Scholars have debated the reasons for this stark reversal—from forbidding the consumption of blood to sanctifying it as a bridge to the divine. Some speculate that, by the time of the Second Temple, many Jews were Hellenized, and early converts to Christianity were merely borrowing standard Dionysian rites. For instance, communal “agape feasts,” in which Christians symbolically ate their god’s flesh and drank his blood, resembled older Greek rituals.

David Biale, a professor of Jewish history at the University of California, Davis, has traced this divergence between Jewish and Christian traditions in his book “Blood and Belief” (2007). He cites a 1376 letter from the mystic Catherine of Siena to a disciple, in which she presciently warns of schisms within the Catholic Church and invokes the Eucharist as a symbol of unity. She argues that Christians, unlike Jews and Muslims, were “ransomed and baptized in Christ’s blood.” The notion of sacred blood as the vehicle for human salvation, Biale writes, justified the “literal interpretation of the Eucharist as dogma, popular celebrations of the Host that spread throughout Europe, and a new cult of blood relics.”

The blood of martyrs was also believed to cure disease. “The Golden Legend,” a thirteenth-century account of the lives of saints, attributed healing powers to the water used to wash the bloodstained clothes of Thomas à Becket, the Archbishop of Canterbury, who was murdered in 1170. The blood of St. Peter Martyr, who was killed by Cathar heretics in 1252, was also accorded medicinal properties. Blood became a central feature of Christian iconography—the stigmata, the Sacred Heart—and also a notable element in Christian anti-Semitism. In the late twelfth century, in England, a spate of pogroms occurred after Jews were accused of murdering Christian children to use their blood in the preparation of Passover matzo. It is tempting to wonder if this calumny, known as the blood libel, is connected in some way with Judaism’s highly exacting rituals designed to avoid the consumption of any kind of blood. The blood libel has proved startlingly resilient, recurring across medieval and early modern Europe, and reappearing in tsarist Russia in the early twentieth century. It has been revived as recently as last year—by Hamas, during its clashes with Israel.