Upton Sinclair was a writer and socialist who wrote The Jungle to dramatize the plight of exploited workers in the U.S. In this book, which was first serialized in a socialist newspaper, Lewis showed how the meat packing industry in Chicago forced workers to toil in harsh, dangerous conditions for very low pay. The workers would be rapidly replaced if they got ill or injured and had no rights.
The book chronicles the downfall of a family who, in addition to toiling in terrible jobs, also tries to pursue the American Dream by purchasing their own home. However, they don't understand the terms of the contract and are soon in over their heads with ballooning payments and a shoddy house. Eventually, they are put on the street.
Lewis wrote the novel in hopes of encouraging support for unionization, worker's protections and decent wages. Instead, middle class audiences were shocked at the depictions of the filth in the meat packing industry. The book led to reforms there, including government inspections of slaughterhouses. As Lewis said, he aimed for people's hearts, meaning he hoped to raise sympathy for the worker, but hit people's stomachs instead.