[shadowgovt]

The mutualism of microorganisms and humans is a fascinating and relatively new sphere of research. One such space is the family of antibacterial (and antiviral) viruses that have apparently evolved into a mutualistic relationship with humans: their development and proclivity is tied to their hosts (they spread by direct placental diffusion from mother-to-fetus, or by milk, they're too fragile to survive in the external environment, and since humans eating humans is rare they don't have other avenues) and so they actually have features that attack harmful bacteria, attack and disrupt harmful viruses, or generate immune-system antagonistic chemical signals to attract our defenses to incoming infections.

They flew under the radar all this time because they're viruses; in general, the only viruses we're aware of in humans are the ones that cause harmful symptoms because actually visualizing the little bastards requires busting out the ol' electron microscope, so if they don't give us a reason to look we aren't looking!