Yes, maggots are creepy, crawly, and slimy. But that slime is a remarkable healing balm, used by battlefield surgeons for centuries to close wounds. Now, researchers say they’ve figured out how the fly larvae work their magic: They suppress our immune system.

Maggots are efficient consumers of dead tissue. They munch on rotting flesh, leaving healthy tissue practically unscathed. Physicians in Napoleon’s army used the larvae to clean wounds. In World War I, American surgeon William Baer noticed that soldiers with maggot-infested gashes didn’t have the expected infection or swelling seen in other patients. The rise of penicillin in the 1940s made clinical maggots less useful, but they bounced back in the 1990s when antibiotic-resistant bacteria created a new demand for alternative treatments. In 2004, the U.S. Food and Drug Administration approved maggot therapy as a prescription treatment.

Although anecdotal reports suggested that maggots curb inflammation, no one had scientifically tested the idea. So a team led by surgical resident Gwendolyn Cazander of Leiden University Medical Center in the Netherlands siphoned samples of maggot secretions from disinfected maggots in the lab and added them to donated blood samples from four healthy adults. The researchers then measured the levels of so-called complement proteins, which are involved in the body’s inflammatory response.

Every blood sample treated with maggot secretions showed lower levels of complement proteins than did control samples — 99.9 percent less in the best case, the team reports in the current issue of Wound Repair and Regeneration.

For good measure, the team tested the maggot secretions again after a day, a week, and a month to determine their shelf life. They also boiled some. To their surprise, the secretions were more effective after boiling and lost no potency after sitting on the shelf for a month.

It’s not surprising that maggot secretions would suppress the immune system, Cazander says. Otherwise, the larvae would probably be attacked by the body.