NEW DELHI — Mahitosh Sarkar came here from his distant village in West Bengal 12 years ago looking for a better life, and he found it. He abandoned the penniless existence of a subsistence fisherman to become a big-city vegetable seller. His wife found work as a maid. Their four children went to school. Their tiny household, a grim but weather-tight room in a dilapidated tenement, had a color TV and a satellite dish.

But these days Mr. Sarkar is counting losses, not blessings. His 10-year-old son died along with more than 70 others when their tenement collapsed on Nov. 15. His wife is in the hospital with a broken leg. All of their possessions, including that color TV, are gone.

The crumbled remains of the illegal building in which the Sarkar family lived in a riverside neighborhood of East Delhi have become an emblem of India’s failure to come to grips with its urban explosion.

After decades of being primarily a nation of farmers, India’s countryside is emptying out, as millions leave their stagnant villages and flock to the cities. But India’s urban infrastructure has not kept pace, and that failing now threatens to undermine the nation’s ability to vault its multitudes out of poverty and share the fruits of its nearly double-digit growth more widely.