[nextos]

This is not necessarily true. Our gut has trillions of bacteriophages. They are actually really useful as they keep bacterial populations under control.

Many bacteriophages are not immunogenic, i.e. our immune system will ignore them. Others are very immunogenic, and are actually used as vaccine platforms.

[rdedev]

Bacteriophages in the bloodstream is different from our gut. Our gut has a lot of bacteria too but the immune system rarely bothers with them till they start popping up outside the guy. We've coevolved with phages for a long time. So our immune system knows that they are mostly benign but don't like them hanging around the bloodstream and flush them out pretty quick