The bacteria of our microbiome are mostly our allies. But they can also they can be turned against us. Two new studies in mice have found that viruses - including one that causes polio, and another that causes cancer - can exploit gut bacteria to infect our bodies.

They use molecules on the bacteria's surfaces as reins, to ride towards host cells, or backstage passes to sneak past the immune system. Our microscopic allies can turn into unwitting collaborators for dangerous infections.