When Upton Sinclair published The Jungle in 1906, the novel became an instant sensation, exposing the horrifying conditions in America’s meat-processing industry. With its stomach-turning depictions of the stockyards and slaughterhouses, the book lit a new fire under the pure food movement and inspired swift passage of landmark food safety laws.

Two years earlier, in the fall of 1904, Sinclair had boarded a train to Chicago in search of material for his Great American Novel. For seven weeks, the 26-year-old writer and devout socialist investigated the dangerous and oppressive working conditions endured by what he called “the wage slaves of the Beef Trust.” Donning grimy clothes and carrying a dinner pail to sneak into Chicago’s “Packingtown,” a dense complex of stockyards, feed lots, slaughterhouses and meat-packing plants, Sinclair was horrified by what he saw.

Titled The Jungle as a metaphor for capitalism, Sinclair’s novel originally appeared in monthly installments between February and November 1905 in the socialist newspaper Appeal to Reason. It tells the story of an idealistic Lithuanian immigrant working for a fictional meat-processing company who loses his family, job, home and health in a succession of calamities before finding hope in socialism.

The Jungle had a limited audience at first, but it became a bestselling sensation when Doubleday, Page & Company published a revised version in February 1906. It sold more than 150,000 copies in its first year.

Much as Uncle Tom’s Cabin had influenced the national conversation about slavery, Sinclair hoped his epic would spark outrage about “wage slavery” and promote socialism as a solution. The Jungle did shock the American public and prompt legislative change—but not in the way he wanted.

The front-page headlines that followed the release of The Jungle did not focus on the oppression of Chicago’s meatpackers, as Sinclair had intended. Instead, they spotlighted revolting details about the meat Americans were eating.

Sinclair splattered The Jungle with blood and guts as he chronicled the unsanitary conditions inside Chicago’s meatpacking plants. As readers turned the novel’s pages, their stomachs turned as well. Sinclair described walls painted with animal blood and plastered with flesh, rotten beef doctored with chemicals and dead rats and sawdust swept into sausage meat. Workers infected with tuberculosis coughed and spat blood onto floors and used open latrines next to processed meat.

The vivid, nauseating descriptions lent momentum to the pure food movement, which had begun in the 1880s with the work of food scientists such as U.S. Department of Agriculture chief chemist Harvey Washington Wiley. Investigative journalists exposed rampant packaging fraud, such as brick dust sold as cinnamon and tinted corn syrup marketed as honey.

“Food manufacturing was unregulated. There was a license to do whatever you wanted to maximize profits,” says Deborah Blum, author of The Poison Squad: One Chemist’s Single-Minded Crusade for Food Safety at the Turn of the Twentieth Century. “There’s nothing that could be done to the food that’s illegal because there [were] no food safety laws.”

And Americans were dying because of it. The chemical-laced “embalmed beef” fed to soldiers during the Spanish-American War proved more lethal than combat. And by one estimate, writes Blum, food and milk tainted with formaldehyde killed 400,000 American infants a year.

Although the public increasingly demanded action, federal legislation to regulate the misbranding and adulteration of food and drugs languished for years. “There’s just no political will in Congress to pass legislation in part because members [were] getting huge amounts of money from food and drink interests not to pass anything,” Blum says. The grisly details in The Jungle eventually broke the logjam.

Just weeks before the publication of The Jungle, President Theodore Roosevelt announced his support for a federal food safety law. Although 100 letters a day in support of food safety legislation poured into the White House after the novel’s release, the law stalled in Congress as Roosevelt summoned Sinclair to Washington, D.C., in early April.

Although both men backed federal food safety laws, they were not natural allies. “Roosevelt had no patience for socialism,” Blum says, “and he was playing a political game that he thought Sinclair didn’t really understand.” At the same time, Sinclair was skeptical that Roosevelt—who had received $200,000 from meatpacking interests for his 1904 presidential campaign and denounced investigative journalists as “muckrakers”—wanted to expose the truth about the meat industry.

Roosevelt assured the author he did. “Mr. Sinclair, I bear no love for those gentlemen, for I ate the meat they canned for the army in Cuba,” the former “Rough Rider” said. Roosevelt told Sinclair he was sending two independent investigators—labor commissioner Charles Neill and social reformer James Reynolds—to inspect Chicago’s stockyards.

Even though the packing houses knew of their visit in advance, the inspectors discovered that conditions were just as bad—or even worse—than depicted in The Jungle. “We saw meat shoveled from filthy wooden floors, piled on tables rarely washed, pushed from room to room in rotten box carts…gathering dirt, splinters, floor filth and expectoration of tuberculous and other diseased workers,” they reported. Neill found Chicago’s stockyards so repulsive, he refused to feed his family meat unless it came fresh from local farms.

The meat industry could dismiss The Jungle as fiction, but not the Neill-Reynolds report. Sinclair knew it would be explosive, and he would leverage it to his advantage.

Believing Roosevelt wasn’t acting fast enough, Sinclair leaked the inspectors’ findings to the New York Times in late May. “Tell Sinclair to go home and let me run the country for a change!” a fuming Roosevelt directed Doubleday, Page & Company.

The investigators’ discoveries further sickened the public and eroded the opposition to a meat safety bill by powerful congressmen such as House Speaker Joseph Cannon. As meat sales dropped in the United States, the impact of The Jungle, which was translated into 17 languages within months of its publication, spread around the world. Germany and France banned American meat products, and British imports of American canned meat ceased.

After releasing an eight-page summary of the Neill-Reynolds investigation in early June, Roosevelt threatened to make the entire report public if Congress did not put a meat inspection bill on his desk. “Roosevelt used the report as a bludgeon, and after these contracts started disappearing, packers went back to Congress and agreed to the Meat Inspection Act,” Blum says.

On June 30, 1906, Roosevelt signed the first comprehensive federal food safety laws in American history. The Meat Inspection Act set sanitary standards for meat processing and interstate meat shipments and prohibited companies from mislabeling or adulterating their products. The Pure Food and Drug Act created the U.S. Food and Drug Administration and prohibited the manufacture or sale of misbranded or adulterated food, medicines and liquor in interstate commerce. Since its 1906 publication, The Jungle has never been out of print. It remains one of the seminal books in American history, a novel that changed the country—just not in the way Sinclair planned. “I aimed at the public’s heart,” he wrote, “and by accident I hit it in the stomach.