I called a friend in Mumbai, Shashank Joshi, who is a member of his state’s COVID-19 task force. “Our I.C.U.s are nearly empty,” he told me. Joshi is a doctor with seemingly infinite reserves of energy: a stethoscope perpetually dangling across his chest, he has spent the past several months carrouselling among slums, hospitals, and government offices, coördinating the state’s response. Early last spring, when the first serious spread of COVID-19 was reported in India, Joshi jumped into action. Dharavi, in Mumbai, is Asia’s largest slum: a million residents live in shanties, some packed so closely together that they can hear their neighbors’ snores at night. When I visited it a few years ago, open drains were spilling water onto crowded lanes. (The next monsoon season, three young boys fell into the drains and died.) The tin roofs of the houses overlapped one another like fish scales; a roadside tap dripped a brown fluid that passed for potable water. When a toddler ran out from an open door onto the street, a neighbor caught him and lifted him up. Someone in the family—I counted six people in a single room, including an elderly couple—sent another child to retrieve him. In that episode alone, I later realized, I had witnessed at least nine one-on-one contacts.