Viruses in the gut protect from infection | Biology & Biotech baubles |
Phages in mucus aid immune system by killing invading bacteria.

...animal mucus — whether from humans, fish or corals — is loaded with bacteria-killing viruses called phages. These protect their hosts from infection by destroying incoming bacteria. In return, the phages are exposed to a steady torrent of microbes in which to reproduce.

Via Ed Rybicki