This psychosis has a name: India syndrome. In 2000, the French psychiatrist Régis Airault wrote the definitive book on the phenomenon, Fous de l'Inde, which means "crazy about India." It relates his experiences as the staff psychiatrist for the French consulate in Mumbai, where he treated scores of his countrymen whose spiritual journeys had taken tragic turns. "There is a cultural fantasy at play," he explains. "[India syndrome] hits people from developed Western countries who are looking for a cultural space that is pure and exotic, where real values have been preserved. It's as if we're trying to go back in time."

Unfamiliar environments have long been known to bring on episodes of short-term delirium. In 1817, the French writer Stendhal described being physically overcome by the experience of viewing Florentine art; a century and a half later, the psychiatrist Graziella Magherini coined the term Stendhal syndrome (also called Florence syndrome) after treating patients who'd become dizzy and confused, even hallucinating or fainting, while visiting the Italian city.

Neither Stendhal syndrome nor India syndrome is listed in the Diagnostic and Statistical Manual of Mental Disorders IV, the bible of psychological illnesses, but 25 "culture-bound syndromes" are. One is called Qigong psychotic reaction. A psychosis whose symptoms include paranoia and visual and auditory hallucinations, it has been observed among practitioners of the ancient Chinese breathing-and-movement discipline Qigong and of extreme variants of yoga like Kundalini. When the intense cognitive effort required in these practices is combined with a strange, possibly frightening new place, it's more likely to result in a mental break.

India syndrome may not be an officially recognized disease, but many doctors are convinced it's real. Kalyan Sachdev, the medical director of Privat Hospital in New Delhi, says that his facility admits about a hundred delusional Westerners a year, many of whom had been practicing yoga around the clock. "There's the physical side of yoga and the psychic side, and sometimes people get it all out of order," he says. "Peaceful people can get aggressive even if they haven't taken any drugs." His treatment tends to be simple: Send them home as soon as possible. "People come to us with acute psychotic symptoms," he says. "But you put them on the plane and they are completely all right." Sunil Mittal, the head of the psychiatric unit at Cosmos Institute for Mental Health & Behavioral Sciences in New Delhi, recently had to send police to retrieve a California woman who'd overstayed her visa and refused to leave an ashram outside Rishikesh. There, Mittal says, she danced erotically in the courtyard each night for the yogis and was often observed in a "trancelike state." His prescription for her was also a return flight home.

Often, however, more than just a plane ticket is indicated. Airault, who currently practices in Paris, recently treated a well-traveled, seemingly stable French optometrist in his thirties who'd begun having feelings of persecution after visiting the holy city of Pushkar—according to him, after drinking a bhang lassi. From there he fled to the countryside, then to Mumbai, where he was found babbling about how the Church of Scientology was telling him to cut himself off from society. Back home in Paris, he was twice institutionalized and spent four years refusing to leave his house. "He was completely crazy, in a state of delirium, a psychosis that was set off by his trip to India," Airault says, adding that, through consultations, the man's condition has improved enough that he can hold down a job at a clothing store. The psychiatrist brushes off the suggestion that the patient might have developed the same problems even if he'd never left France. "It's important to understand that sometimes we go crazy in India because it's a culture too different from our own," he says. "It doesn't mean that we're mentally ill."

• • •

Beginning in 1997, I lived in India off and on for more than a decade. Westerners whose journeys had taken a wrong turn were commonplace there. The most notorious was Gary Stevenson, a Texan supposedly descended from Robert Louis Stevenson, who, after joining the Aghori—a group of wandering holy men who demonstrate the renunciation of physical and material attachments by covering themselves in cremation ash—could often be seen on the streets of Rishikesh begging for alms, using a human skull for a bowl.

Did India make him come unglued, or was he already unstable? There's no way to know for sure. But in 2006, I met a Westerner who I'm certain did suffer from India syndrome. I was guiding a group of American college students on a trip through the spiritual centers of Delhi, Varanasi, and Dharamsala (where the Dalai Lama lives in exile). The highlight was a 10-day silent-meditation retreat in the town of Bodh Gaya, where the Buddha attained enlightenment two millennia ago. The lectures were based on the Tibetan tradition of Lamrim, or "steps on the path." One student, whom I'll call Emily, was a preternaturally calm 21-year-old from an upper-class Catholic family who'd practiced yoga since high school. At the retreat, she sat in a flawless lotus position during three-hour daily meditation sessions led by a bald Swiss-German anee, or nun, in crimson robes. Emily maintained her vow of silence without apparent difficulty, spending much of her free time writing in her journal.