Most of the viruses present in people’s guts are bacteriophages, but how they interact with resident bacteria is still an open question.

Antibiotics disrupt the immune response to the influenza vaccine in people who haven’t recently had exposure to the virus or immunization.

Physical activity, independent of diet, shifts the composition of bacteria in the intestines, spurring researchers to search for species that might provide benefits akin to working out.

In Niger, fewer kids die when health workers administer azithromycin routinely, perhaps because of a shift in the composition of gut microbes.

Curbing the growth of harmful bacteria in mouse microbiomes reduces the animals’ incidence of inflammation-related colorectal cancer.

Through their effects on metabolism and immunity, bacteria in the gut affect whether medications will be effective for a given patient.

Calories aren’t enough to correct malnourishment, but eating foods that spur specific microbes to grow in the gut can.

Animal studies and a few small clinical trials show it’s possible to get commensal microbes to protect against bone loss, rather than contribute to it.

Research in mice and humans is beginning to establish a link between the composition of microbes in the gut and immune responses to tumor cells, but the mechanisms are not yet clear.