A key problem with cancer therapies has always been distinguishing healthy cells from cancerous ones, and viruses are exquisitely good at recognizing specific cells to attack. Erholtz's cancer, called myeloma, allows the buildup of malignant blood plasma cells in her bone marrow. Measles viruses happen to have the ability get into bone marrow.
Her doctors at the Mayo clinic injected her with a genetically engineered version of the weakened virus used in measles vaccines. The dose needed to be huge, so that her immune system would not kill the viruses before they could kill the cancerous cells. That exposes a weakness of this specific therapy: it probably could not work in patients with immunity to measles already. Erholtz and the one other patient in this trial did not, but most of us in the U.S. are vaccinated for measles at a young age. Symptoms from the measles infection itself disappeared after a few weeks for the two patients.
The other myeloma patient also did not achieve complete remission like Erholtz, possibly because her tumors formed in the muscle rather than the bone. And of course, the result would need to be confirmed in larger trials to prove it wasn't a one-time fluke. Such a clinical trial is expected to start in September.
Other researchers in the field have deployed different viruses to treat different cancers, such as a variant of the common cold virus for pancreatic cancer. It'll be years before viral therapies for cancer become routine—if the results even hold up in clinical trials—but this suggests we could eventually deploy viruses for the good of our health, too.