In late 2016, a group of plaintiffs’ lawyers took the stage at the year’s largest gathering of their colleagues to talk up a promising new target.

For 30 minutes, they laid out arguments linking the popular weedkiller Roundup to cancer. An arm of the World Health Organization had pegged Roundup’s main chemical ingredient as a probable carcinogen the year before, and it was quickly becoming a focus of the plaintiffs’ bar.

Some product-liability lawyers in the audience in Las Vegas were skeptical. Tying exposure from everyday products like Roundup to cancer often is less straightforward than linking illness to medications or medical devices, said Chase Givens, a lawyer with the Cochran Firm who attended the event. But the presenters’ track records mounting complex cases got the audience’s attention.

Three years later, more than 42,700 farmers, landscapers and home gardeners have sued Bayer AG, Roundup’s manufacturer, claiming the company knew the herbicide posed a cancer risk but failed to warn consumers. Bayer is contesting the lawsuits and argues that scientific research and regulatory reviews, including from the Environmental Protection Agency, prove Roundup’s safety.

Behind the surge in lawsuits is a little-known, sophisticated legal ecosystem that includes marketing firms that find potential clients, financiers who bankroll law firms, doctors who review medical records, scientists who analyze medical literature and the lawyers who bring the cases to court.