Viruses normally seem like evil little germs; they often invade our bodies’ cells and hijack their inner machinery to mass produce viral invaders, which eventually destroy their host cells as they bust out to besiege more cells. The process repeats ad nauseam. But what if that wicked master plan could be twisted to do our bidding? What if viruses only invaded cells we wanted them to kill—like cancer cells?

Researchers got the initial idea of using viruses decades ago (PDF) and racked up a number of enticing anecdotes that suggested the idea could work. After all, cancer cells, in their altered state that allows them to replicate quickly and uncontrollably, often shut off their anti-viral defenses and are prone to infection. But scientists’ attempts at training viruses to be cancer assassins fell short in bigger trials—until now.

On Tuesday, the Food and Drug Administration approved the first viral-based cancer therapy. The virus, called talimogene laherparepvec (T-VEC), is a genetically engineered herpes simplex virus that can help treat advanced melanoma. Its genetic tweaks make it incapable of usurping normal, healthy cells. Thus, it doesn’t cause herpes. But when it meets cancer cells, it initiates cellular carnage. The virus is also engineered to carry a gene encoding the GM-CSF protein, which triggers the immune system. As the virus ravages cancer cells, GM-CSF gets released, boosting a patient’s natural defenses.

So far, T-VEC isn’t a wonder drug, but it’s helpful. In a large clinical trial published in May in the Journal of Clinical Oncology, researchers found that the virus shrank tumors and extended patient survival by a median of 4.4 months. The results were just shy of showing a statistically significant benefit to overall health. Still, the researchers are hopeful that when the viral therapy is combined with traditional therapies, it could significantly boost survival rates in patients.

And with T-VEC’s approval, researchers see a clear path for more viral-based remedies. Many such therapies are already in the works, and researchers are combing through the large varieties of viruses for the next potential cancer slayer.