Every living thing on the planet plays host to viruses, and bacteria are no exception. Bacteriophages—or just “phages” to those in the know—are the viruses that attack bacteria. And we are in dire need of things that attack bacteria, since many pathogenic bacterial species have developed resistance to the antibiotics we’ve thrown at them for decades.

Phage therapy is attractive not only because antibiotic use yields antibiotic resistance but also because the treatment can be targeted specifically to the bacteria causing an infection. Most antibiotics in use are rather broad-spectrum, so they obliterate many of the bacteria they encounter, including the ones that are happily residing in our guts, minding their own business and not causing anyone any problems. Phages can be more precise.

But is phage therapy effective? A new study suggests it may end up being undercut by our own immune systems, which treat the therapies like a hostile invader.