NEW DELHI — Rajni Gill woke up with a slight fever in mid-April, the first warning that she had Covid-19. Within a few days, she was breathless and nearly unconscious in a hospital.

Desperate to arrange plasma treatment for Ms. Gill, a gynecologist in the city of Noida, her family called doctors, friends, anyone they thought could help. Then her sister posted a plea on Facebook: “I am looking for a plasma donor for my sister who is hospitalized in Noida. She is B positive and is 43.”

The message, quickly amplified on Twitter, flashed across the phone of Srinivas B.V., an opposition politician in nearby Delhi, who was just then securing plasma for a college student. He deputized a volunteer donor to rush to the blood bank for Ms. Gill.

“The administration and systems have collapsed,” Mr. Srinivas said. “I have never seen so many people dying at the same time.”

“Mine and my team’s work may be a drop in the ocean — but a drop nevertheless,” he said.

With India’s health care system overwhelmed by India’s unprecedented Covid surge, which is bringing about 400,000 new cases and thousands of deaths each day, desperate relatives and friends of the infected have resorted to sending S.O.S. messages on social media. And many of those calls are being answered.

Some people need medical oxygen, which is nearly impossible to find in Delhi, the capital. Others are hunting for medicine that goes for high prices on the black market, or for ventilators that are exceedingly rare.