Bacteriophages — viruses that infect and kill bacteria — are more diverse in people with Crohn’s disease or ulcerative colitis. Herbert “Skip” Virgin, an immunologist at Washington University in St. Louis, and colleagues reportthe finding online January 22 in Cell. In particular,Caudovirales bacteriophages were more diverse in people with the diseases than in healthy people living in the same households, the researchers found.

The viruses may kill friendly bacteria in the intestines, leading to inflammation and disease. Scientists already knew that people with Crohn’s and colitis tend to have fewer types of beneficial bacteria in their gut microbiomes. The microbiome is the complete collection of bacteria, viruses, fungi and other microorganisms that live in or on the body. What researchers hadn’t understood is why the bacterial diversity decreased.

The new results suggest some viruses may damage health not by infecting human cells, but by altering the microbial mixes in the body. The findings may also explain why fecal transplants from healthy people generally haven’t cured inflammatory bowel diseases; lingering viruses may destroy newly introduced bacteria before they can do the patient any good.