In 1891, a New York doctor named William B. Coley injected a mixture of beef broth and Streptococcus bacteria into the arm of a 40-year-old Italian man with an inoperable neck tumor. The patient got terribly sick—developing a fever, chills, and vomiting. But a month later, his cancer had shrunk drastically. Coley would go on to repeat the procedure in more than a thousand patients, with wildly varying degrees of success, before the US Food and Drug Administration shut him down.

Coley’s experiments were the first forays into a field of cancer research known today as immunotherapy. Since his first experiments, the oncology world has mostly moved on to radiation and chemo treatments. But for more than a century, immunotherapy—which encompasses a range of treatments designed to supercharge or reprogram a patient’s immune system to kill cancer cells—has persisted, mostly around the margins of medicine. In the last few years, though, an explosion of tantalizing clinical results have reinvigorated the field and plunged investors and pharma execs into a spending spree.

Though he didn’t have the molecular tools to understand why it worked, Coley’s forced infections put the body’s immune system into overdrive, allowing it to take out cancer cells along the way. While the FDA doesn’t have a formal definition for more modern immunotherapies, in the last few years it has approved at least eight drugs that fit the bill, unleashing a flood of money to finance new clinical trials. (Patients had better come with floods of money too—prices can now routinely top six figures.)

But while the drugs are dramatically improving the odds of survival for some patients, much of the basic science is still poorly understood. And a growing number of researchers worry that the sprint to the clinic offers cancer patients more hype than hope.

When immunotherapy works, it really works. But not for every kind of cancer, and not for every patient—not even, it turns out, for the majority of them. “The reality is immunotherapy is incredibly valuable for the people who can actually benefit from it, but there are far more people out there who don’t benefit at all,” says Vinay Prasad, an Oregon Health and Science University oncologist.

Prasad has come to be regarded as a professional cancer care critic, thanks to his bellicose Twitter style and John Arnold Foundation-backed crusade against medical practices he says are based on belief, not scientific evidence. Using national cancer statistics and FDA approval records, Prasad recently estimated the portion of all patients dying from all types of cancer in America this year who might actually benefit from immunotherapy. The results were disappointing: not even 10 percent.

Now, that’s probably a bit of an understatement. Prasad was only looking at the most widely used class of immunotherapy drugs in a field that is rapidly expanding. Called checkpoint inhibitors, they work by disrupting the immune system’s natural mechanism for reining in T cells, blood-borne sentinels that bind and kill diseased cells throughout the body. The immune cells are turned off most of the time, thanks to proteins that latch on to a handful of receptors on their surface. But scientists designed antibodies to bind to those same receptors, knocking out the regulatory protein and keeping the cells permanently switched to attack mode.

The first checkpoint inhibitors just turned T cells on. But some of the newer ones can work more selectively, using the same principle to jam a signal that tumors use to evade T cells. So far, checkpoint inhibitors have shown near-miraculous results for a few rare, previously incurable cancers like Hodgkin’s lymphoma, renal cell carcinoma, and non-small cell lung cancer. The drugs are only approved to treat those conditions, leaving about two-thirds of terminal cancer patients without an approved immunotherapy option.