This extraordinary development wouldn’t have been possible without animal research. Yet many in the animal-rights community condemn the use of any and all animals in medical research and continue to push for testing bans. Such efforts ignore the fact that when it comes to medical research, animal models are indispensable. Further proof of this came on Wednesday with news in the journal Nature that a drug to fight Ebola had showed remarkable success when tested in rhesus monkeys.

The brain-tumor treatment developed at Duke is a re-engineered polio virus. The new virus designed by researchers helps the body’s immune system to recognize and attack cancer cells. As in countless other revolutionary therapies, animal research played an invaluable role in creating this treatment.

Before human trials began, the re-engineered virus was injected into the brains of macaque monkeys, whose systems operate similarly to those of humans. Since the raw polio virus often results in paralysis, such testing of the modified virus made sense—and helped demonstrate that the body’s immune system would cripple brain tumors if injected with the re-engineered virus.

This wasn’t the only instance where animal models proved crucial for the Duke team. While developing their therapy, these researchers relied on years of previous primate research.

One such study was a 1991 paper in which Harvard researchers used a genetically engineered virus to treat a mouse with GBM. In 1996 researchers at the State University of New York at Stony Brook used mice to prove that infecting a cell with a polio virus required a specific receptor on the cell’s surface. Then, in 2000, a research team from Duke and Stony Brook showed how a genetically modified polio virus eliminated human tumors bearing that special receptor in mice. This discovery laid the groundwork for the clinical trials that resulted in this breakthrough therapy.

Dependence on animal research is hardly unique to Duke researchers. A number of recent medical advances have their roots in animal models. Consider a Phase III clinical trial from 2013, which proved that a next-generation herpes virus could successfully treat melanoma patients. This research was the direct result of a 1995 study by scientists at the University of Pennsylvania and Georgetown University, demonstrating how a modified herpes virus can shrink tumors in mice and nonhuman primates.

More recently, animal research has helped pave the way toward restoring vision. Last September, a Japanese woman became the first person to undergo an experimental stem-cell treatment for blindness. The procedure was deemed safe for humans after several studies involving monkeys and mice.

The greatest medical contributions from animal research may still lie in the future. In a study published last year in the journal Stem Cell Reports, scientists in France and Germany were able to regenerate damaged brain areas in mice for the first time. The discovery could lead to treatments for human brain damage caused by everything from strokes to bullet wounds.

Despite these successes, critics continue to attack animal-research methods as needlessly cruel. Activists have succeeded in pressuring all but one major airline to stop carrying animal models to research labs. That’s a problem for scientists in the U.S. Most monkeys come from Asia and Mauritius, where they’re humanely raised on farms.

Consequently, researchers have had to turn to charter carriers. As a result, costs per animal have tripled. Those extra costs sap medical progress.

Another common argument by critics is that animal models rarely lead to discoveries that are relevant to humans. It’s undeniable that human physiology differs from that of mice or monkeys. But humans and animals still have much in common. Primates share fundamental similarities—from their use of hormones to their reactions to infection—that, for centuries, have helped deepen our understanding of the human body.

With the Duke trials, the project director initially called the idea of using polio as a therapy “nuts” because of the risk of paralysis. Animal models are what enabled his team to move forward with their work.

Activists calling for the elimination of animal studies grossly underestimate the human value of animal studies. Those who doubt this value need only look at the faces of patients in the Duke trial whose lives have been saved by these essential research techniques.