A few that come to mind immediately: 1) Phage are highly specific - there's really no such thing as a "broad spectrum phage", which means you need fairly inten

A few that come to mind immediately: 1) Phage are highly specific - there's really no such thing as a "broad spectrum phage", which means you need fairly intensive diagnostic laboratories + an extensive library of phage. 2) There's some regulatory hurdles - these are less than they once were, and the fecal transplant stuff for C. diff, and the treatments that have come from that, have done a lot of how do we think about regulating a living biological treatment. 3) Phage has mostly been used for wounds and other infections where it's relatively easy to get them "on site". For something like sepsis, which is systemic, or some other infections, it's not clear how to get them where they need to go in the concentration they're needed. The solution that most immediately presents itself, which is just "A lot of phage delivered IV, and then hope they multiply when they reach the right sites" involves being able to grow up a lot of phage, which is a non-trivial problem for a lab. There's purification steps that need to happen, you have to have good enough cultures of the infecting organism to grow the phage, etc. It's a very, very cool technology, but it's also something that's been "on the cusp" since I was an undergrad, and I'm a tenured faculty member now.