One of the world's most dreaded viruses has been turned into a treatment to fight deadly brain tumors. Survival was better than expected for patients in a small study who were given genetically modified poliovirus, which helped their bodies attack the cancer, doctors report.

It was the first human test of this and it didn't help most patients or improve median survival. But many who did respond seemed to have long-lasting benefit: About 21 percent were alive at three years versus 4 percent in a comparison group of previous brain tumor patients.

Similar survival trends have been seen with some...