The wild success of the Ice Bucket Challenge, one of the past decade’s feel-good social-media sensations, obscured its origins as a form of extortion. Its source, though murky, might be traced to Norway, where early nominees were pressured to choose between jumping into a frigid body of water and taking care of the weekend’s bar tab. By early 2014, the moral ante had been upped: the penalty for noncompliance became, rather more nobly, a contribution to medical research. This structure—which incentivized bravado at the expense of charity—persisted until the spring, when American firefighters helped repackage the experience. Now a high-pressure hosing was the reward for a donation to a colleague’s cause of choice. That July, a golfer decanted water on his own head in an effort to cheer up his cousin’s husband, who had amyotrophic lateral sclerosis, or A.L.S., and invited others to donate to the “A.L.S. Foundation.” By the end of the month, the idea, now connected to one disease, had taken on a life of its own. Bill Gates engineered an elaborate self-soaking contraption, posted high-definition footage, and challenged Elon Musk. The taunt was at once socially contagious, righteous, and fun. But there were still some disposed to see it as a threat. President Barack Obama, when drafted by Ethel Kennedy, Justin Bieber, and Donald Trump, declined in favor of a hundred-dollar contribution to A.L.S. research.