Bacteria are all around —and inside—us. Some are harmless, some are beneficial and some, of course, cause disease. Others, such as the common bacterium Streptococcus pneumoniae, defy categorization. They are turncoats, with the ability to suddenly switch from good to bad.

Usually the microbe dwells harmlessly in people's nasal passages. Every so often, however, when S. pneumoniae senses danger, it disperses to other areas of the body in a bid to save itself, making us sick. The bacteria cause serious disease, such as pneumonia, which is the leading cause of childhood death worldwide, “but they are accidental pathogens,” says Anders Hakansson of the University at Buffalo.

Evidence has shown a strong correlation between illness from influenza and subsequent infection by S. pneumoniae, but just how the bacterium turns virulent has remained murky. So Hakansson and his colleagues set out to investigate.

They found that the change seems to be triggered by the human immune response to the flu. When the body responds to a flu virus by raising the temperature and releasing stress hormones such as norepinephrine, the bacteria react in turn to those changes in their environment. They disperse from their colonies and begin expressing different genes that make them more deadly to respiratory cells, the team reported in mBio.

The ability of S. pneumoniae to intercept hormones and other distress calls from human cells exemplifies a phenomenon called interkingdom signaling—in this case, a bacterium tuning in to signals from the animal kingdom—which is gaining recognition as a key biological mechanism. Hakansson notes that because S. pneumoniae is native to humans, it makes sense that the microbe would have evolved ways to read changes in the landscape. Says Hakansson: “We are the bacteria's ecological niche.