You can roughly guess a person’s age based on how they know John Green. Older millennials met Green and his brother, Hank, on YouTube in the late 2000s, when they were performing songs about Harry Potter and cultivating a fan base of “Nerdfighters.” Younger millennials probably know him as the author of “The Fault in Our Stars,” “Looking for Alaska” and other best-selling young-adult novels that feature complex and frequently tragic teenage characters. Generation Z-ers herald Green as a beloved professor: Since 2012, he has hosted video lessons on history, literature and religion for an educational web series called “Crash Course.”

Members of Generation Alpha, born after 2010, now comprise many of his 2.8 million followers on TikTok. Fans are drawn to the brothers’ inside jokes and eclectic interests, like a holiday centered on a photo of Green with a mustache, or an annual charity event called the Project for Awesome (put on by the Foundation to Decrease World Suck). But even dedicated Nerdfighters were surprised by Green’s intense focus on eradicating tuberculosis, a disease that kills 1.6 million people annually. TB is caused by a bacterium that is estimated to infect one-quarter of the world’s population, but the disease receives little attention in wealthy nations.

Green has emerged as an unlikely spokesperson in the global effort to fight the disease. His latest project, a book called “Everything Is Tuberculosis,” interweaves the social and scientific histories of tuberculosis with the present-day story of a young man from Sierra Leone named Henry Reider. The book will be published in March by a division of Penguin Random House.

In a recent interview, Green described the book as his attempt to understand how tuberculosis could be both entirely curable and the deadliest infectious disease in the world. It explores what the disease’s persistence as a scourge “says about us as a species — what that says about the worlds we’ve chosen, the world we might choose instead,” he said.

In countries like the United States, where testing and treatments are readily available and the prevalence of the disease remains low, tuberculosis is rarely mentioned outside of global-public-health labs and classroom discussions of 20th-century literature. But it thrives where tests and medication are expensive or otherwise inaccessible, funding is lacking and people are malnourished and live in poorly ventilated quarters.

Green said that he aims to bridge the “empathy gap” that exists for the disease, which disproportionately affects people in sub-Saharan Africa, Eastern Europe and Asia.

“A disease like TB needs advocates who have access to the megaphones of the contemporary internet, as it’s experienced in rich countries,” he said. “Part of the reason we allowed so many millions of people to die of TB over the last decade is because we didn’t know we were allowing it. We weren’t paying attention to the crisis.”

Henry and Shreya

Green’s interest in TB arose in much the same way as he wrote, in “The Fault in Our Stars,” about falling in love: slowly, and then all at once. He had read about the disease, but it wasn’t until a 2019 trip to Sierra Leone that he experienced tuberculosis up close. Green and his wife, Sarah, had spent their visit touring maternal clinics and hospitals supported by Partners In Health, a nonprofit co-founded by the physician Paul Farmer. (Green is a trustee of the organization.) On the last day, guides took them to Lakka Government Hospital, the country’s only dedicated tuberculosis facility.

“It had been neglected, and like the rest of the health care system in Sierra Leone, was deeply impoverished,” Green recalled.

There, he bonded with Henry, a friendly boy who shared his own son’s name. Henry was “one of those preternaturally charismatic young people who just feels comfortable and walks you around and shows you everything,” Green said. Assuming that Henry was a staff member’s child, Green asked the doctors about him and was told that he was being treated for multidrug-resistant tuberculosis.

“He’s 16 years old, and the only reason you think he’s 9 is because he’s so emaciated by tuberculosis,” Green was told.

As Green soon learned, Henry was one of many patients who desperately needed access to bedaquiline, a lifesaving drug manufactured by Johnson & Johnson. Another was Shreya Tripathi, a 19-year-old TB patient in India. In 2017, Tripathi successfully sued her country’s government for access to the medication, but she died in 2018 from irreversible lung scarring. A doctor who provided expert testimony in Tripathi’s case later told Green that in her final months, Tripathi read and reread “The Fault in Our Stars,” his novel about terminal cancer patients.

The story stuck with Green. In 2023, after being asked to speak at a high-level meeting at the United Nations, he noted that Tripathi had been alive at the previous iteration of the meeting.

“She didn’t really die of tuberculosis,” he said. “She died of a flawed patent system. She died of government bureaucracy. She died of a court system that moved too slowly.”

Green’s initial foray into advocacy for tuberculosis treatment came last July. He posted a video on YouTube berating Johnson & Johnson for its role in inflating the price of brand-name bedaquiline. “Tell your friends about this injustice,” he implored viewers. “Tell your family. Tell the internet. Because the only reason Johnson & Johnson executives think they can get away with this is that they think we aren’t paying attention in the part of the world where they sell most of their products.”

“When I was making that video,” Green said recently, “I was thinking about Shreya, I was thinking about Henry.”

Beyond philanthropy

The call to action was viewed more than 1.4 million times, and days later Johnson & Johnson agreed to let generic manufacturing proceed for the drug, effectively cutting the treatment’s price from $1.50 to $0.50 per day. Green pointed out that the decision culminated “a very, very long process” between Johnson & Johnson, Partners In Health and Doctors Without Borders. Even so, he added, the timing was notable.

“Johnson & Johnson certainly went to extraordinary lengths to argue that they signed, sealed and delivered this deal X number of weeks before my video,” he said. “But I can’t help but notice that they didn’t announce it until after.” A Johnson & Johnson spokesperson said that the company’s action was taken “to address the misperception that access to our medicine was limited or restricted.”

Carole Mitnick, a professor of global health and social medicine at Harvard Medical School and a senior research associate at Partners In Health, described Green as a welcome voice. “I don’t think anybody has ever captured the attention of such a large number of people when talking about TB until John entered the scene,” she said. “Maybe it’s just more of us screaming into the great void, but I do have to believe that having more voices raising attention about these issues can contribute to changes.”

Green generated another social media storm last September with a series of videos and graphics on YouTube, TikTok and X pressuring the makers of a TB diagnostic test to lower the price of its test cartridges. A week later, the test’s makers agreed to reduce the price of the cartridges by 20%. Stijn Deborggraeve, a diagnostics adviser for Doctors Without Borders, said that global health groups like his had mounted similar pressure for years, to no effect.

Other celebrities including Angelina Jolie and Brad Pitt have donated large sums to TB eradication efforts. But Green said that he sought to go beyond philanthropy by engaging his fans, the Nerdfighters. In a 2009 video, Green cheekily defined a Nerdfighter as “a person who, instead of being made out of, like, bones and skin and tissue, is made entirely of awesome.” What began as an online fandom has morphed into an organized collective that takes part in real-life meetups and annual charity drives. So far in 2024, the Nerdfighters have raised $3.5 million for nonprofits including Save the Children and Partners In Health.

“Our online community is extraordinary,” Green said. “And I don’t mean that in a braggy way; I mean that they’re unusual.” His videos about diagnostic testing and “patent evergreening” — when a pharmaceutical company acquires a patent on a slightly tweaked drug formula to prevent generics from entering the market — are avidly consumed. In turn, his views, likes and comments shape the content that Green creates. “The truth is that they pull us as much as we pull them,” he said.

Zoe Millspaugh, 22, a Nerdfighter who recently graduated from the University of Texas at San Antonio with a degree in public health, said: “The more we drum up knowledge about the disease, the more that people understand about it, and the more that people can help.”

Mitnick first met Green last year. Since then, she said, the two have struck up a productive working relationship — she has answered questions he has about the disease and fact-checked the numbers he has cited in his videos.

“He’s a storyteller,” she said. Compared with an academic like her, she added, “he’s freer to just say, ‘Look, I’m a human who cares deeply about this planet and the other people who live on it.’”

For his part, Green said that he tried to keep the focus on “the right people”: researchers and doctors on the front lines. “I do try to make it clear that I am in no way the hero of the fight against tuberculosis,” he said. “The heroes in the fight against tuberculosis are working at Lakka.”

Henry Reider, who is now 21 and features prominently in Green’s book, said that he was excited for his story to reach a wider audience. He counted “Looking for Alaska” among his favorite novels and credited Green with inspiring him to post his own YouTube and TikTok videos about water scarcity in Sierra Leone.

The book’s title, “Everything Is Tuberculosis,” comes from a phrase that his wife often levels at him to characterize what has become a yearslong obsession with the disease, Green said. Another ongoing project has him advocating for the passage of the End Tuberculosis Now Act, which would authorize the president of the United States to provide international assistance in eliminating TB. And he returned to YouTube last month to renew pressure against the tuberculosis diagnostic test manufacturer, which has yet to publicly release the results of a promised audit. (A spokesperson for the company said that it had conducted an assessment showing that the new price did not exceed the cost of producing the test cartridges.)

“I don’t see this book as an ending or as a form of closure in my relationship with TB,” Green said. “I very much see it as a new beginning in my relationship with the disease and supporting the activists who are trying to end it.”