Every 12 years, the northern Indian city of Allahabad plays host to a vast gathering of Hindu pilgrims called the Maha Kumbh Mela. This year, Allahabad is expected to host an estimated 80 million pilgrims between January and March. (See Kumbh Mela: Pictures From the Hindu Holy Festival)

People come to Allahabad to wash away their sins in the sacred River Ganges. For many it's the realization of their life's goal, and they emerge feeling joyful and rejuvenated. But there is also a darker side to the world's largest religious gathering, as some take advantage of the swirling crowds to abandon elderly relatives.

"They wait for this Maha Kumbh because many people are there so nobody will know," said one human rights activist who has helped people in this predicament and who wished to remain anonymous. "Old people have become useless, they don't want to look after them, so they leave them and go."

Anshu Malviya, an Allahabad-based social worker, confirmed that both men and women have been abandoned during the religious event, though it has happened more often to elderly widows. Numbers are hard to come by, since many people genuinely become separated from their groups in the crowd, and those who have been abandoned may not admit it. But Malviya estimates that dozens of people are deliberately abandoned during a Maha Kumbh Mela, at a very rough guess.

To a foreigner, it seems puzzling that these people are not capable of finding their own way home. Malviya smiles. "If you were Indian," he said, "you wouldn't be puzzled. Often they have never left their homes. They are not educated, they don't work. A lot of the time they don't even know which district their village is in."

Once the crowd disperses and the volunteer-run lost-and-found camps that provide temporary respite have packed away their tents, the abandoned elderly may have the option of entering a government-run shelter. Conditions are notoriously bad in these homes, however, and many prefer to remain on the streets, begging. Some gravitate to other holy cities such as Varanasi or Vrindavan where, if they're lucky, they are taken in by temples or charity-funded shelters.

In these cities, they join a much larger population, predominantly women, whose families no longer wish to support them, and who have been brought there because, in the Hindu religion, to die in these holy cities is to achieve moksha or Nirvana. Mohini Giri, a Delhi-based campaigner for women's rights and former chair of India's National Commission for Women, estimates that there are 10,000 such women in Varanasi and 16,000 in Vrindavan.

But even these women are just the tip of the iceberg, says economist Jean Drèze of the University of Allahabad, who has campaigned on social issues in India since 1979. "For one woman who has been explicitly parked in Vrindavan or Varanasi, there are a thousand or ten thousand who are living next door to their sons and are as good as abandoned, literally kept on a starvation diet," he said.

According to the Hindu ideal, a woman should be looked after until the end of her life by her male relatives—with responsibility for her shifting from her father to her husband to her son. But Martha Chen, a lecturer in public policy at Harvard University who published a study of widows in India in 2001, found that the reality was often very different.

Chen's survey of 562 widows of different ages revealed that about half of them were supporting themselves in households that did not include an adult male—either living alone, or with young children or other single women. Many of those who did live with their families reported harassment or even violence.

According to Drèze, the situation hasn't changed since Chen's study, despite the economic growth that has taken place in India, because widows remain vulnerable due to their lack of education and employment. In 2010, the World Bank reported that only 29 percent of the Indian workforce was female. Moreover, despite changes in the law designed to protect women's rights to property, in practice sons predominantly inherit from their parents—leaving women eternally dependent on men. In a country where 37 percent of the population still lives below the poverty line, elderly dependent relatives fall low on many people's lists of priorities.

This bleak picture is all too familiar to Devshran Singh, who oversees the Durga Kund old people's home in Varanasi. People don't pay toward the upkeep of their relatives, he said, and they rarely visit. In one case, a doctor brought an old woman to Durga Kund claiming she had been abandoned. After he had gone, the woman revealed that the doctor was her son. "In modern life," said Singh, "people don't have time for their elderly."